Monday, February 15, 2010

Curry Puff Problem

The PSLE2009 item (see Page 32 PSLE Examination Questions 2005-2009) available from several publishers or the Popular bookshop in Singapore is a good illustration of when calculator use is expected and when calculator should not be used.

The problem is about Super Puff Shop where you can buy one curry puff for 80 cents but when you buy three curry puffs, you get to buy one more at half the price. The problem is to find out the gretaest number of curry puffs that one can buy with $50.

You do not expect 12-year-olds to use calculator to find half the price of 80 cents. That should be an automatic number fact. Students are also not expected to use any computational tool - whether it is a calculator or paper and pencil - to find the cost of three puffs. In fact, to find the cost of a set of four curry puffs should be done mentally. Although it is best to write down how one arrives at the value and this can be done by writing down the number sentence 3 x 80 cents + 40 cents = $2.80, the actual computation should be done mentally.

In order to find how many complete sets of 4 curry puffs one can buy, $50 divided by $2.80 is appropriate. This computation should be done using calculator. We should not want kids in the present technological age to waste time (and be frustrated in the process) to compute 50 divided by 2.8 using long division - it is simply not productive.

It is better to use the time to teach them that although the calculator shows a bunch of digits (17.85714...), only the whole number part (17) is of any meaning in the present situation.

That means 17 x $2.80 = $47.60 is used to buy 17 x 4 puffs or 68 puffs. I would expect most people to use calculator to do the former and do not need one to do the latter. 17 x 4 is easy to do using the doubling strategy - double 17 to become 34 and double 34 to become 68.

There is still a remainder of $2.40 - no 12-year old should need a calculator to know the change for $47.60 when one pays $50. We don't use a calculator in real life for this situation. Most adults know what the change is in this situation.

The remainder gets you another 3 puffs. Does that need the use of a calculator? We would have failed if kids need a calculator to work that out!

There is a difference between having a calculator to use and using a calculator indiscriminately.


  1. If you give a child a calculator, what is to prevent him or her from using it indiscriminately? With the calculator in hand, why bother with the doubling strategy for 17 x 4. Or to subtract 47.60 from 50. And even with 50/2.8 it is possible to simplify to 12.5/0.7, which is easy to do. The sooner you make calculators available, the sooner they will use them for even computations that can be solved with some number sense strategies.

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