
Direct and Proportion
Hi,
Im trying to do this question
$\displaystyle y$ is directly proportional to $\displaystyle x$ and $\displaystyle y = 21$ when $\displaystyle x = 7 $. Find the value of $\displaystyle y$ when $\displaystyle x = 13$
$\displaystyle y = \frac{k}{x} = 21$
$\displaystyle = \frac{k}{7}$
$\displaystyle = 21 X 7 $
$\displaystyle = 147 $
Now put 147 into the formulae with $\displaystyle x$
$\displaystyle y = \frac{147}{13}$
$\displaystyle = 0.something $
Answer book says 39, how?
Thanks

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

Thanks. I should read the question in future (Giggle)