What will the depth of water in the tank be after 17 days?

Question

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.

Answer 1

The goes down by 6 inches a day. (63 - 57) / 2

how much does the water go down in the next 12 days?

 

Note: the answer is impossible. Check the data in the problem!