Final answer:
Walter will mow 4/7 of an acre in an hour.
Step-by-step explanation:
To find out how many acres Walter will mow in an hour, we need to find out how many acres he mows in 7/10 of an hour and then divide that by 7/10.
Given that he mows 2/5 of an acre in 7/10 of an hour, we can calculate:
2/5 / 7/10 = 2/5 × 10/7 = 20/35 = 4/7
Therefore, Walter will mow 4/7 of an acre in an hour.