Jack usually mows his lawn in 5 hours. Marilyn can mow the same yard in 7 hours. How much time would it take for them to mow the lawn together?

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Answer 1 · 2018-06-21T08:11:24

$approx 2.92$ hours
$approx 2$ hours $55$ mins

If Jack can mow his lawn in $5$ hours, it means that we can mow $1/5$ of his lawn in $1$ hour.

Similarly, Marilyn can mow $1/7$ of the same lawn in $1$ hour.

So, together, they can mow $1/5+1/7 = 12/35$

This means that after $t$ hours they can mow $12/35 t$ lawns.

We want to know how many hours it takes them to mow one lawn, so we want to solve

$12/35 t = 1$

and thus

$t = 35/12 approx 2.92$

And $2.92$ hours are approximately $2$ hours and $55$ minutes.

Jack usually mows his lawn in 5 hours. Marilyn can mow the same yard in 7 hours. How much time would it take for them to mow the lawn​ together?