SOLUTION: Jack usually mows his lawn in 4 hours. Marilyn can mow the same yard in 5 hours. How much time would it take for them to mow the lawn together?

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Question 345074: Jack usually mows his lawn in 4 hours. Marilyn can mow the same yard in 5 hours. How much time would it take for them to mow the lawn together?
Answer by nerdybill(7384) About Me  (Show Source):
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Jack usually mows his lawn in 4 hours. Marilyn can mow the same yard in 5 hours. How much time would it take for them to mow the lawn together?
.
Let x = time (hours) it takes for both
then
x(1/4 + 1/5) = 1
multiplying both sides by 20 we get:
x(5 + 4) = 20
9x = 20
x = 20/9
x = 2.22 hours
or
x = 2 hours and 13 minutes