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Cereal Percentages

March 13, 2019 Leave a comment

This week my Y11s are sitting mock exams. One of the questions that came up on paper 1 stumped a lot them.

They came out if the exam on monday, and said the paper was very difficult. One of them asked me one of the questions:

“Sir, if you have a box of cereal and increase it by 25% but keep the price the same, what percentage would you need to decrease the price of the original box by to get the same value?”

I immediately said “20%”, an answer which flummoxed the student and the others stood around. They couldn’t work out how I had got that answer, never mind so quickly.

I tried to explain it to them, but in that moment, on the corridor, I didn’t do a very good job. For me, it was intuitive. A 25% increase and a 20% decrease would yield the same value as in one you are changing the top of a fraction and the other the bottom of a fraction so you need to use the reciprocal, 4/5 is the reciprocal of 5/4 and 4/5 is 80% hence it needs to be a 20% decrease. Cue blank looks and pained expressions. I was seeing the students again later in an intervention session so I promised to go through it in more detail then.

I talked about the idea of value, how you could consider mass/price and get grams per penny – how many grams for each penny you spend – or you could consider price/mass and get penny per grams – how much you pay per gram. I said either of these would give an idea of value and you can use either in a best value problem.

I showed them the idea of the fraction, said you could call the price x and the size y.

The starting scenario is:

y/x

The posed scenario is:

1.25y/x

but we know 1.25 is 5/4 so that becomes:

(5/4)y / x

which in turn is:

5y/4x

I then showed that the second scenario meant getting to the same value but altering x. To do this you would need to mutiply x by 4/5:

y/(x(4/5))

(y/x)÷(4/5)

(y/x) × (5/4)

5y/4x

This managed to show some of them what was going on, but others still massively struggled. I tried showing them with numbers. 100 grams for £1. This again had an effect for some but still left others blank.

I’m now racking my brains for another way to explain it. If you have a better explanation, please let me know in the comments of via social media!

Categories: #MTBoS, GCSE, Maths Tags: , , ,
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