y is directly proportional to x When x= 15, y = 10 .What is the value of y when x = 21

Question

I don't understand anything in this topic , well i understand some things but not much. I was on the internet looking for a answer till I saw this site well I realise in some countries they use y=kx, but in my country we se y=ax

Answer 1

Direct variation is y = kx

The given numbers form one ratio expressd as y/x =10/15

To find when x = 21

We set up another ratio y/21

By the definition both ratios are equal 10/15 = y/21

Multiple each side by 21.

(2/3)21 = (y/21)21

14 =y

y = 14