It takes my father 3 hours to plow our cornfield. It takes me 5 hours to plow our cornfield. If we both plow for 1 hour, how long will it

Question

It takes my father 3 hours to plow our cornfield. It takes me 5 hours to plow our cornfield. If we both plow for 1 hour, how long will it take him to finish the plowing?

Answer 1

Father does 1/3 per hour

Son does 1/5 per hour

 

Together in an hour they do 1/3 + 1/5 = 5/15 + 3/15 = 8/15

This means there is 7/15 of the field left to plow

 

(1/3 per hour)(x hours) = 7/15

x = 3(7/15) = 7/5 = 1 2/5 hours or 1 hours 24 mins