Okay, so the rate is 1 job / hours, so:
1/3 + 1/7 (This covers both their work rates) = 1/n (n represents the time it take for them to work together)
Get the LCD:
7/21+ 3/21 = 1/n
10/21 = 1/n
Cross multiply:
10n = 21
n = 2.1
It will take them 2.1 hours, or 2 hours and 6 minutes
