MHB Are you smarter than a Singaporean 10 year old?

[GLONE007]

So i was looking through The Guardian(newspaper) and found this question, Titled 'are you smarter then a 10 year old?' and as you can guess i had no clue what to do. Also i thought if i looked at the answer maybe i would realize how it worked. Oh. how wrong was I! Is anyone able to break it down even more?. so that someone like 'me' can understand how it is done and how it works.

1. Mary cut off 2/5 of a piece of string. Later, she cut off another 14 m. The ratio of the length of string remaining to the total length cut off is 1 : 3. What is the length of the remaining string?

A. 5 m
B. 7 m
C. 10 m
D. 14 m
Solution is C.

I saw two ways of doing this yet they both through me off, i was wondering if someone could. in a sense talk it through step by step.As i am determined to understand how it works.

Many thanks.

 

[MarkFL]

Re: Are you smarter then a Singaporean 10 year old?

what we wind up with is (where $\ell$ is the original length in m):

$$0.6\ell-14=\frac{1}{3}\left(0.4\ell+14\right)$$

$$1.8\ell-42=0.4\ell+14$$

$$1.4\ell=56$$

$$\ell=40$$

Hence:

$$\frac{3}{5}40-14=24-14=10$$

edit: I've moved this thread here, as it's a better fit.

 

[mitaka90]

I believe I can explain it well, because tbh is quite easy to understand if you check out my way. The simplest thing you could've done is to express the total length of the string. You can use any letter you want, but usually l stands for length in Maths as Mark has used.

\ell=