For this you need to set up a ratio. Sasha can do you 3/8 in 45 minutes, which is 3/8:45, or (3/8)/45. In an hour, she can go x/60. Set the two equal to each other:
(3/8)/45 = x/60
Cross Multiply:
(3/8)*60 = 45*x
45x = 22.5
x = 1/2
She can do half an acre in an hour
