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

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Y = mx

15 = m (3)

m = 5

when y = 4

4 = 5 (x)

x = 0.8 or 4/5

Y varies directly as x means:

y α x

y = kx where k is constant of proportionality

y/x = k

when y = 15, x = 3, y/x = k

15/3 = k

5 = k

k = 5.

Now when y = 4, x = ?

Recall: y/x = k, k = 5

4/x = 5

5x = 4

x = 4/5

Therefore x = 4/5

I hope this explains it.