For questions related to direct proportion, recall that you let y be equal to the product of a constant k and the quantity that it y is proportional to. In this case, you let y = k (x^3). Substitute in the values to find the value of k. Use the expression you obtained for y to solve part (b).

For 17(c), let the value for x substituted be in terms of x, in this case, 1.2x. Then find the percentage change.

Question 18 uses the same approach as question 17.