Suppose y varies directly as x and y=27 when x=3. Find x when y = 45.
'Varies directly' most likely means an equation of the form y = x + k
Where k is a constant. So as x increases by some amount, y increases by that same amount.
change in y = change in x
y2 - y1 = x2 - x1
45 - 27 = x2 - 3
18 = x2 - 3
21 = x2
when y = 45, x = 21