The first answerer's answer 35 minutes must be incorrect because it is obviously too much time; since Lisa alone takes 30 minutes to shovel the driveway, she and anyone else together would take less than 30 minutes (and 35 > 30). Averaging the times is an invalid method.

Correct solution:

Lisa can shovel 1/30 of the driveway each minute.
Tom can shovel 1/40 of the driveway each minute.
Together, they can shovel 1/30 + 1/40 = 7/120 of the driveway each minute.
Therefore, together they take 120/7 = 17 and 1/7 minutes to shovel the whole driveway.

Lord bless you!

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