Oh please help me with my maths marking, I am being so thick..

[Caz10]

Disclaimer - it's a Friday night, I'm very very tired, I've been marking for 2hrs, I;ve got my period....I'm not this stupid normally...

Joe packed a large suitcase. It weighed 40kg. The case was 25% overweight. By how much was his suitcase overweight?

I think I am overthinking this - is it as simple as 25% of 40? I am thinking that I have to try to work out what original weight plus 25% of said weight would = 40kg. Pupil has just given 10kg as answer. I have no answer book. I am tired. Etc.

Also I know my 5 times table better than my 125 so could work out 40/5!

The correct answer is 8kg.

caz10 - whilst the maths of 40/5 4 is obviously easier - the reason for doing it isn's as clear. we only do that because we know that it's the same as 40/125 100. You would need to be able to explain why this is so, otherwise 40/5 4 appears completely random. basically what you are doing if you do 40/54 is working out 25 percent of the target weight (instead of one per cent) and then multiplying by 4 to get to 100 percent (instead of by 100). which, imo, is harder to explain and less helpful in terms of understanding what percentages are all about. and understanding what they're all about is the most important thing - once you grasp that then you can muck about and simplify the maths.

The reciprocal fraction in this kind of question is one fifth or 20%. The easy way to answer it is to ask the question what is 20% of the whole ie 40kg, so 8 kg. There are 5 25% chunks of weight in the suitcase.

125% = 40kg
25% = 40kg/5 = 8kg

Thanks guys. Will re-read your posts when I am less tired!

Isn't it 40 x 25/125? So a fifth? Or 8kg overweight.

As one who just spent three hours marking Year * English tests I just wanted to offer some sympathy X

Sympathy gratefully accepted! Saturday afternoon, DD asleep, so it must be marking time again...