Answer:
Since Keitaro walks at a pace of 3 miles per hour and runs at a pace of 6 miles per hour, we can use the following system of inequalities to represent the number of hours he can walk, w, and the number of hours he can run, r, to reach his goal:
3w + 6r ≥ 36
3w + 6r ≤ 90
The first inequality states that the total number of miles Keitaro walks and runs must be at least 36 miles per month. The second inequality states that the total number of miles Keitaro walks and runs must not exceed 90 miles per month.