Hello!
So the other night at D&D my DM threw out a "Math Riddle" and I couldn't remember how set up the solution. Please help!

Alice and Bob take 2 hours to do a job.
Alice and Charlie take 3 hours to do the job.
Bob and Charlie take 4 hours to do the same job.
How long would it take all three of them to do the same job working together?

A+B=2
A+C=3
B+C=4
A=.5
B=1.5
C=2.5
It's obvious Alice is under paid and Charlie and Bob should be fired, but I just couldn't remember how o set up to solve the last part of the problem.

I've been out of school for 15 years so I lost a few less used skills along the way.