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Doubling And Halving When Buying T-Shirts

Doubling and Halving When Buying T-Shirts

Last November, we discussed the use “doubling and halving together” (https://www.math-inic.com/blog/doubling-and-halving-together/) to simplify some multiplication problems. We wrote that the technique works well when one factor is even and the other ends in 5. We also gave other examples where doubling and halving are useful.

Yesterday, while I was browsing thru my books to look for topics to write for this week’s newsletter, I came across a story found in page 22 of my book, 25 Math Short Cuts. (You can purchase this book from the MATH-Inic Philippines FB page: https://www.facebook.com/MATHInicPhils  or thru any of our affiliates).

This story is another example of using doubling and halving which I have not discussed in my previous article but which I have personally experienced:

During the Philippine Science High School’s ANAC I (A National Alumni Convention I) where I represented Pisay Dos (Batch ’70), I saw a booth selling Pisay T-Shirts. I was attracted to a nice red shirt and though of buying one for myself. Then I thought of my five children who are all Pisay alumni: Rose (class 1990), Gil (class 1992), Eman (class 1995), Angel (class 1997) and Nica (class 1999). I selected 5 more of their sizes. Then I remembered Rheea, Gil’s wife who is also his batchmate. And so I chose another shirt for her.

When I asked the ladies manning the booth (who are both Pisay alumni) the cost of a shirt, they told me it was P350.

As one of the ladies took her calculator, I blurted out “P4,900 divided by 2 is P2,450.” They were amazed after she punched in 350 x 7 in her calculator, that she got P2,450.

“How did you do that?” she asked me.

“I doubled P350 to get P700, which when you multiply by 7 will give you P4,900. Now dividing this by 2 will give you P2,450.”

By mastering doubling and halving, I expanded my multiplication table.  

Attached is a picture of us proudly wearing our red Pisay T-Shirts.

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