# Math Help - Direct and Proportion

1. ## Direct and Proportion

Hi,

Im trying to do this question

$y$ is directly proportional to $x$ and $y = 21$ when $x = 7$. Find the value of $y$ when $x = 13$

$y = \frac{k}{x} = 21$

$= \frac{k}{7}$

$= 21 X 7$

$= 147$

Now put 147 into the formulae with $x$

$y = \frac{147}{13}$

$= 0.something$

Thanks

2. ## answr

Hi!

The problem says it is directly proportional so:

y=kx

if it were inversely proportional it would be y=k/x as you wrote.

Now if you calculate as you were doing but with the above correction you will arrive to k=3 and eventually you will obtain y=39 for x=13

3. Thanks. I should read the question in future