If x varies directly as y, and x = 48 when y = 16, find x when y = 5.

X directly varies with y which means as x increases, y also increases. In this case, we represent a constant of proportionality of k.

Applying laws of  proportionality, 

x =k y
48 = k* 16
k = 3

when y is 5
x = k y
x = 3* 5 
x = 15