Answer: y = 20
Reason
y varies directly as x, so y = kx for some constant k.
y = 15 and x = 3 pair up to get k = 5 because of the scratch work below.
y = kx
k = y/x
k = 15/3
k = 5
The equation y = kx becomes y = 5x.
Then plug in x = 4.
y = 5x
y = 5*4
y = 20
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Another approach
y/x = y/x
15/3 = y/4
15*4 = 3y ....... cross multiplication
60 = 3y
3y = 60
y = 60/3
y = 20 is the final answer.