Book Maths Puzzle Algebra

A group of men did a job in 30 min. another group of men did it in 20 min. IF both groups worked together how long will it take to finish the job?

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Book Other Question

One mile upstream from his starting point, a rower passed a log floating with the current. After rowing upstream for one more hour, he rowed back and reached his starting point just as the log arrived. How fast was the current flowing??
How do I determine if this ordered pair (0,2) is a solution of the following equation: -3X + Y = -2? Thanks in advance.