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The choc bar starter: proportion

The chocolate bar did not have the weight printed anywhere we could see. So the student ('A' to protect the innocent as there is a diet involved here) snapped the label and sent me a copy.

There is enough information in the picture above to find the weight of the bar. I used this as a starter in another Foundation GCSE lesson, I just printed out several colour copies of the image and asked "can you find the weight".

Discussions around

• Why kcal?
• The kcal per 100g figure makes it like percentages
• The bar has 221 kcal and that is less than half the 539 kcal in a 100g serving, so bar must weigh less than 50g
• Some prompting lead to 221/539 * 100 as the weight, 41.002g or 41g given the rounding in the kcal figures

I'm going to try this in a Higher GCSE lesson as follows:

If the kcal figures are rounded to the nearest whole kcal, what is the smallest and largest weight consistent with the figures?

I make it

• Smallest: 220.5/539.5 * 100 = 40.87g
• Largest: 221.5/538.5 * 100 = 41.13g

Not that much variation.

Keith Burnett, Last update: Tue Apr 01 2014