Can you solve this tricky 2017 PSLE Math question on ribbons?

Think first.

Mandy How | October 02, 2017, 06:39 PM

Primary school exam questions are getting notoriously tough.

This time, a rather tricky Math question for this year's Primary School Leaving Examinatons (PSLE) has made it to the headlines again.

Yahoo News reports the question as such (note that this is not the exact phrasing found in the paper) :

Jess wants 200 ribbons of length 110 cm for a party. However, the ribbons were sold at 25 m per tape. How many tapes will Jess need?

At first glance this looks relatively straightforward, if one knows his metrics well (1m = 100cm, for those of us who are less mathematically inclined).

Some might think the following steps are enough:

1) Finding the total length of ribbon needed: 110cm x 200 = 22,000cm

2) Converting the length of a single tape to metres: 25m x 100 = 2,500cm

3) Dividing the total length of ribbon needed by the length per tape: 22,000cm/2,500cm = 8.8 tapes.

4) Feeling pleased that you remember to round it up, because 8 tapes won't have sufficient length, duh: 9 tapes needed.

But 9 is not the correct answer. Welp.



But this method is erroneous as it neglects the fact that each tape is not divisible by the length of ribbon needed. In other words, there will be leftover ribbon on each tape that cannot be used.

Therefore, the correct method should be:

1) Divide the length of the tape by the length of a ribbon to find out how many ribbons can each tape can produce: 2,500cm/110cm = 22.7 ribbons. Round it down to 22 ribbons per tape, as 0.7 is not a complete ribbon.

2) Divide the amount of ribbons needed by the amount of ribbons produced per tape, to find out how many tapes she needs: 200/22 = 9.09 tapes.

3) Since 9 tapes are not enough, one will have to purchase 10 tapes, which is the correct answer.

Did you get that right?


Time for more tuition. Thank you shadow education industry.

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