The Chocolate Factory: Place Value in Algebraic Thinking

Even good math students may begin studying algebra with deficiencies in their understanding of place value. The following activity offers middle school students concrete, hands-on experience with the concept of place value and an opportunity to express the concept algebraically. In the process, students clarify their understanding of the difference between numeral and number while enhancing their number sense. Hopefully students will be able to replace the blind procedure of borrowing and carrying with genuine understanding.

Although students may easily name the place-value column for any particular digit in a number, they often do not understand the significance of the names and they cannot explain mathematically why regrouping works. Students need help in formulating a mathematical explanation to replace the non-math explanation they usually hold. In an activity called “The Chocolate Factory”, students pack chocolates and then tally the number of boxes and cases.

Practically speaking, I usually use beans instead of chocolate. I tell students they are packaging chocolates as the chocolates come down an assembly line. Each piece of chocolate is a “one” or a unit. Students pack the pieces into boxes of ten pieces each, then pack the boxes into cases of ten boxes each, keeping a tally in a table:

I give students opportunities to gain experience adding and subtracting “Cases,” “Boxes” and Loose Chocolates.” Another advantage to using the column names as shown in the table is that the activity can be recycled to teach any base. I have found it is more helpful to rename the “ones” place “loose chocolates”. Then it is easy to explain that there are loose chocolates when there are not enough chocolates to fill a box. There will never be 10 “loose chocolates”, because 10 will fill a box, thereby adding 1 to the tally in the “Boxes” column.

If the above table represents base 10, converting the above tally to numerals yields 562. Students readily understand that as they accumulate 10 boxes, they transfer those boxes as 1 case and put a tally mark in the “Case” column. Often at this point the light bulbs go on, and students see the concept of carrying for the first time. We continue packing chocolates and "carrying" for a while. Then we "eating" the chocolates, metaphorically speaking. Perhaps starting from the above tally, I ask them to take out 8 pieces of chocolate. They will naturally want to open a box to accomplish this. As they take a box, they erase a tally mark and dump the 10 chocolates (beans) with the loose chocolates, resulting in a total of 12 tally marks in the “loose chocolates” column. This activity is very similar to other trading activities used to teach place value, but seems to be more effective at building the concept of place value because we avoid giving the columns numerical names at the outset.

Simultaneously, we keep a record of each subtraction in the standard algorithm. Students will often understand regrouping for the first time as they compare the physical packing of the chocolates with the ongoing mathematical representation. Three things are going on at the same time, the packing activity, the data record in terms of a tally and the data record in terms of the standard algorithm. We expand and repeat the activity with other groupings which I have carefully planned in advance. For example, if we repeat with groupings of 7, then 7 chocolates make a box, and 7 boxes make a case. I give the students 289 chocolates, knowing full well they will again end up with 5 cases, 6 boxes, and 2 "loose ones". Sometimes I like to use the word "leftovers" for"loose ones."

Students instantly want to know how they got the same apparent number, 562. In the ensuing class discussion, we talk about why the first 562 (10 to a box) has more chocolate pieces than the second 562 (7 to a box). We discover that the reason we line up columns for addition and subtraction is not merely for neatness sake, but because the grouping determines the mathematical meaning of the columns. Students find they can work just as readily in other bases as long as they remember the basis of the grouping (pun intended). I prefer to name the columns from right to left in base ten as: loose, 10, 100, 1000 etc. If, we are working in, for example, base 7, the column names are "loose", 7, 49, 343, etc. Eventually, we can work in base y with column names y^0, y^1, y^2, y^3, etc. As the Chocolate Factory activity illustrates, the “ones” are “ones” only because there are not enough of them to make the next grouping level. They are the ungrouped loose ones, whether in base ten or any other base.

The Vital Place of Place Value

Perhaps one of the most important foundational concepts in mathematics is place value. As the Massachusetts Department of Education rightly observes, “The subtly powerful invention known as place value enables all (my emphasis) of modern mathematics, science, and engineering. A thorough understanding removes the mystery from computational algorithms, decimals, estimation, scientific notation, and—later—polynomials” (Massachusetts Department of Education (2007). In fact, it is when students first meet polynomials in algebra, that the lack of a proper grounding in place value becomes painfully apparent. Most likely a significant number of the difficulties that students experience with math may be traced to place value.

I reviewed the state standards of various states with regard to place value. I looked for an explicit reference to “regrouping,” the current term for what we used to call “borrowing” and “carrying.” My survey of state standards resulted in a mixed bag. Some states require students to do little more than name the place value of a particular digit. Other states expect students to use various means to model place value. Alaska asks students to not only perform the operations of addition and subtraction, but to explain those operations.

State standards have their utility, but apparently whatever the specific state standard, students are able to follow the regrouping recipe without having any real understanding of why the recipe works. In fact, adults of all ages add and subtract by mindlessly following the recipe. Most adults, and of course, all children could do with a solid grounding in place value.

I have a number of activities I use to make place value explicit. Tomorrow I will tell you about an activity I like to call “The Chocolate Factory.”

Are You Good at Non-Math?

One of the most persistent issues in math education has been the reliance on non-mathematical explanations of mathematical principles. For example, we tell students that when multiplying positive and negative numbers “two negatives make a positive.” Such an explanation clarifies nothing about how the numbers behave or why an ostensibly English grammar rule should apply to math.

What is worse, we tell students who successfully master such non-math explanations that they understand math, or that they are good at math, when really what they are good at is the blind procedures of non-math. Young children have no way to distinguish non-math from math. They believe, because we have told them, that they are learning math, when in fact they are learning non-math. If it does not catch up to them earlier, it often catches up to them in algebra class where historically “A” students may find themselves inexplicably failing to understand the subject material.

Children rely on adult teachers to initiate them into the joys and delights of math, but often teachers make math a difficult subject, usually because they themselves understand non-math rather than math. After all, if numbers are running around, it must be math, right? Even sadder are the number of elementary teachers who lack an interest in acquiring what math education researcher Liping Ma called “the profound understand of fundamental mathematics” even while believing that they “know” math.

Many colleges of education and community colleges have sought to address the serious weaknesses in the mathematical understanding of elementary teachers by either requiring, or at least offering, coursework in mathematics for elementary teachers. I am quite sure a survey of professors teaching such required courses would report remarkable levels of student resentment at being forced to take a class in something they think they already know, to “jump hoops” as they say . Some of these students may wake up and get motivated to learn the math concepts. Some seethe inwardly as they pass the class. However, most students will pass the class and eventually be certified to teach regardless of their poor attitude toward or lack of understanding of the vital core subject of mathematics.

Only later, once they are in the classroom, will they be likely to regret the squandered opportunity to finally get math. Perhaps they may grow to appreciate the professor who tried to give them the gift of mathematical understanding, a gift they resisted at the time.