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Lisa can shovel her drive way in 35 minutes. Tom can shovel the same drive way in 40 minutes. How long will it take them to do the job together?

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in one minute, Lisa can shovel 1/35 of the driveway

in one minute, Tom can shovel 1/40 of the driveway.

in one minute, they both can shovel 1/35 + 1/40 = 8/280 + 7/280 = 15/280 = 3/56 of the driveway.

So, it will take them 18 2/3 minutes working together.

If b is the time for both,

1/b = 1/35 + 1/40

b = 35*40/(35+40) = 18 2/3

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