Question

Y varies directly with x, and y = 15 when x = 3. What is the value of x when y = 4?

Answer 1

Y varies directly with x so use
y/x

so when y=15 and x=3

substitute =15/3

when y=4 use y/x again so 4/x (you do not know the x)

15 4
--- * ---
3 x

cross multiply
15x=12
isolate x by dividing 15 on both sides
get x=12/15

Answer 2

When x = 3 , y = 15 , divide both by 3.
When x= 1 , y = 5. Now multiply both by 4/5
When x = 4/5 , y=4