math
posted by Anil .
A lawn sprinkler covers a rectangular pattern of 20ft x 30ft.
1)If the flow rate of the sprinkler is 2.5 gpm, derive a formula for calculating the time (hours) to apply D inches of water over one 20ft x 30ft area.
2)What are the units on the constant?

20ft*30ft*1in = 600ft^2in
1 gal = 231 in^3
2.5gal = 577.5 in^3
1 ft^2in = 144 in^3, so
2.5gal/min = 4.04 ft^2in/min
or, .24935 min/ft^2 in
that's 149.61 min/600 ft^2in
or, 2.4935 hr/600 ft^2in
so, you will need roughly 2.5D hours to cover the area with D inches of water
since your formula is time(hr) = k*D(in), k must be in hr/in