The Kwon family has a rainwater catchment system they use to water their garden. After 3 days without rain, the depth of water in the tank is 63 inches. After 5 days, the depth is 57 inches. What will the depth of water in the tank be after 17 days?

63-57 = 6 inches in 2 days

6/2 = 3 inches per day

17-5 = 12

12*3 = 36 inches in 12 days

57-36 = 21 inches after 17 days

F (d) = 72 - 3d

if d = 3

f (d) = 72 - 3d = 72 - 3 (3) = 72 - 9 = 63 inches

if d = 5

f (d) = 72 - 3d = 72 - 3 (5) = 72 - 15 = 57 inches

so

if d = 17

f (d) = 72 - 3d = 72 - 3 (17) = 72 - 51 = 21 inches

answer

the depth of water in the tank will be 21 inches after 17 days