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

y directly proportional to x is represented by y=ax, where a is the constant of proportionality.

So, 10=15a by substituting for x and y using the given values. Therefore a=10/15=⅔, meaning that y increases two-thirds as much as x increases. The equation becomes y=2x/3, and when x=21, y=2×21/3=14.