We are given that Joan mows 6 acre in 2 hours, to know the time needed by her to mow 3 acres, we will simply do cross multiplication as follows:
6 acre .......... take ......... 2 hours
3 acre .......... take ..........??
time needed = (3*2) / (6) = 6/6 = 1 hour
This mean that Joan will need 1 hour to mow 3 acre