Maths Help...Please

[Bewildered]

DS1 always attacks maths questions in the most peculiar ways, but 99% of the time ends up getting the answer correct!
With the following question his method falls short, I can understand his reasoning but cannot explain why it doesn’t work, and he has asked me to ask on the forum, can any of you maths wizards help, Please?

Put the following in order, starting with the one which is best value.
a) 20g for £2 b) 25g for £3 c) 30g for £4.20
d) 10g for £1.30 e) 24g for £2.64 f) 40g for £3.60

I showed him how I would of found the value of 10g for each and correctly get the answer F,A,E,B,D,C, but he's arguing that his method should work

He used a) as the base thus he got 10g=£1, he then converted this base value to the others, and worked out the difference…
a) 10g=£1 base value so no difference 0p
b) 25g=£2.50 therefore there’s 50p difference from £3
c) 30g=£3 therefore there’s £1.20 difference from £4.20
d) 10g=£1 therefore there’s 30p difference from £1.30
e) 24g=£2.40 therefore there’s 24p difference from £2.64
f) 40g=£3.60 therefore there’s -40p difference for £3.60

His answer was therefore
F,A,E,D,B,C

D and B are in the wrong order, all the rest are correct. Why ?

[Guest55]

You can't just look at the differences as they are relative to different amounts of money.

So the 50p difference is smaller compared to £3 than the 30p is compared to £1 - he is just lucky that the others work!

[Bewildered]

Thanks Guest55

That's initially what I said that they are different amounts, so should be looked at individually, but then got my self confused, because 4 out of 6 were correct, so started thinking hold on a minute... perhaps?

Thanks

BW

[fm]

I recognise this question from the Bond maths books and I am fairly certain the easiest way of finding the answer is to work out what 1g is. I believe the author is hoping a child would have a 'feel' for numbers and notice all the amounts are easily divisible
e.g. 24 g cost £2.64 so 1g = 11p

Certainly when I do this question with pupils I suggest they look at the figures before they decide what technique to use. I would have thought finding 10 g would be much harder.