Posted by **Anil** on Saturday, March 31, 2012 at 3:44pm.

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?

- math -
**Steve**, Saturday, March 31, 2012 at 7:49pm
20ft*30ft*1in = 600ft^2-in

1 gal = 231 in^3

2.5gal = 577.5 in^3

1 ft^2-in = 144 in^3, so

2.5gal/min = 4.04 ft^2-in/min

or, .24935 min/ft^2 in

that's 149.61 min/600 ft^2-in

or, 2.4935 hr/600 ft^2-in

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

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