we can form a ratio equation. the ratio would be the amount of original apples to the cost versus the new amount of apples and new cost. we know all amounts except original cost.
pounds/cost = new pounds/new cost
9/x = (3/4)9 / 15
9/x = (27/4) / 15
9/x = 27/60
9/x = 9/20
cross multiply
9x = 180
x = 20