45 minutes is 3/4 hour. This means that an hour is a third longer (1/4 hour) so we need to multiply 3/8 by 1 1/3 or 4/3: 3/8*4/3=1/2 so Sasha can mow 1/2 acre of grass in one hour.
The rate is therefore 1/2 acre per hour. To check this we multiply 3/4 (of an hour) by 1/2=3/8 acre.