to pick forty bushels of apples?
Let's use the formula:
1/T = 1/T1 + 1/T2
where T is the time it takes both Castel and Willies to pick forty bushels of apples, T1 is Castel's time, and T2 is Willies' time.
Substituting the given values:
1/T = 1/13 + 1/15
Simplifying:
1/T = (15 + 13) / (13 x 15)
1/T = 28 / 195
T = 195 / 28
T ≈ 6.96 hours
Therefore, it would take Castel and Willies approximately 6.96 hours to pick forty bushels of apples if they worked together.