Mike can plow a field in 12 hours and his brother Phil can plow it in 15 hours. How long will it take them to plow it if they use two plows and work together?

6.66 hours, that is, 6 hours and 40 minutes.

Step-by-step explanation:

If Mike can plow a field in 12 hours, we plows 1/12 of the field each hour.

If Phil can plow a field in 15 hours, we plows 1/15 of the field each hour.

So if they are working together, they can plow (1/12) + (1/15) of the field each hour, that is:

(1/12) + (1/15) = (5 + 4) / 60 = 9 / 60 = 3 / 20 = 1 / 6.66

So if they work together, they can plow the field in 6.66 hours, that is, 6 hours and 40 minutes.