y = k/x, k = 80, and y = 80/x
a.The best way to solve this problem is using reverse proportionality, in which two magnitudes x and y are inversely proportional when there is a constant k. If y varies inversely with x, then y = k/x
b. If y = 20 when x = 4, then k = (x)(y) -----> k = (4)(20) = 80
c. y = 80/x