Help with fractions question.

[SouthWiltsGrammer]

I'm trying to help DD and I don't know how to do this question. Probably very simple but I'm not very strong in Maths.

A)Given that Tom eats 3/5 of a cake and his friend Jerry eats 1/3 of the cake what fraction of the cake is left?

How do I work this out?

B)Given that Tom's piece of cake weighed 150 g what was the weight of the whole cake?

Many thanks!

[Sally-Anne]

A) Multiply the lower numbers 5 & 3 to get the Lowest Common Multiple = 15.

You then need to express the fractions as 15ths.

3/5 = 9/15
1/3 = 5/15

The remainder of the cake was therefore 1/15.

B) You now have 9/15 (Tom) and 6/15 (Jerry plus the remaining 15th).

The Lowest Common Factor of 9 & 6 is 3.

9/15 = 150g
LCF: 3/15 is therefore = 50g
6/15 is therefore = 100g

9/15ths (150g) + 6/15ths (100g) = 15/15ths (250g, the weight of the whole cake.)

[Guest55]

For part (b) you could say 3/5th is 150g so 1/5 is 50g.

So the whole cake is 5 x 50 = 250g.

[SouthWiltsGrammer]

Thank you so much, it looks like I need to add common denominators on the ever growing list of things to look out for/revise. I need just as much revision as she does! Seems like a long time since I sat GCSE Maths, embarrassing that the 11+ stuff seems hard to me!

[Guest55]

I think it would be useful to start with 3/5 + 1/5

Then move onto 3/8 + 5/16 and explain fractions need to have the same denominator before they are added or subtracted. So 3/8 needs to be changed to an equivalent fraction with a denominator of 16.

The idea of LCM is confusing if they don't understand why ...