Addition

26 01 2010

We worked on addition today with the aid of my aunt’s 1979 math textbook, Mathematics Revealed. Well, that and a box of Cheerios. Cheerios and other boxed cereals are usually expensive, and they are a rare treat in our household, so they are great for homeschooling. We learned the meaning of the word “sum” and “equation” and the remarkable role of zero in addition. (It does nothing!) We also used the Cheerios as counters to do complicated sums: 23 + 14; 15 + 8. Having practiced on the Cheerios, we worked our way up to expanded forms of addition, partial sums, and introduced the idea of “carrying.” In the end, Floppy could do and check a great big fancy sum like 708 + 99 + 132.

To learn the commutative law of addition, we played store. I made a sheet of play dollar bills, and hired Floppy to be my employee. We made believe he worked a two-day workweek; Monday and Tuesday. On Monday, I hired him to pick up his birthday cards and arrange them on our side table, for a day’s wages of $3. He did this task very willingly, and then I told him to take his money home and go to bed. Then the alarm clock rang: Tuesday had arrived. On Tuesday, I hired him to pick up his play scissors and a mask he had made, and put them away in his room. This was harder work, so his day’s wages was $5.

“How much money did you make this week, Floppy?”

“$8”

“Well, it just so happens it costs $8 in my company store to buy a pile of Cheerios for a snack.” (I hold my son in debt bondage; please nobody teach him the tune to Sixteen Tons.) Crunch, crunch, crunch: A satisfied customer.

Next workweek arrived: He earned $5 on Monday for putting away his dirty laundry, some clean laundry, and a library book. He earned $3 on Tuesday for putting away some scraps from a craft project and papers from school (I’m liking this math lesson more all the time).

“How much money did you make, Floppy?”

“$8”

“But how can that be? Last week I paid you $3 on Monday and $5 on Tuesday and you made $8. This week I paid you $5 on Monday and $3 on Tuesday; but you still earned $8?”

“Mama; as long as you use the same amounts, it doesn’t matter when you pay me. The numbers can change places and the sum is the same!”

And that’s the commutative law — the textbook taught us the Latin derivation, and a bit more about the associative law, which we also demonstrated. Next time we’ll do some practice exercises.

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