The 2nd one is really really easy.
x^2 = 920
x = 31 or something
920 + 920 = c^2
C= square root of 1840 = also the diameter
radius = square root of 1840 divided by 2
pi * r^2 = total area sprinkled
total area sprinkled minus 920 is your answer to the 2nd part.

Sorry I wrote this up real quick its not extremely clear. Ask any questions you have and I'll clarify.