Combining work rates (other ways to solve these problems?)

How do you solve the fractional expression in the first problem?
What other ways are there to solve this type of problem?

Q: If Tom can do a job in 10 hours and Tom and Mary can do the job in 6 hours working at their respective rates, how long will it take Mary to do the job by herself.

I know of two possible ways to solve this.

(1) Using the (X*Y)/(X+Y) formula I came up with:

(10*M)/(10+M) = 6

Where T=X=10 and Y=M. And because my math skills are rusty I don't know where to go from here. Am I cross multiplying the fractional expression?

(2) Using the Princeton method of pie charts:

Tom = w=120
r=12/hr | t=10hrs


Tom and Mary = w=120
r=10 + M? | t=6hrs


Mary = w=120
r= 10/hr | t= 12hrs

I got the question right after plugging in any Work value that is a multiple of 6 and 10; 60, 120, etc.