GMAT Quantitative Section | Formula for multiple rate problem explanation

Since the work done is same throughout the question so for simplicity work done can be taken as 1
Rate*Time = Work
Let Rate of A be A ,Rate of B be B and Rate of C beC

If A can finish a work in X time and B can finish the same work in Y time then both of them together can finish that work in (X*Y)/ (X+Y) time.

A can finish a work in X time
=> A * X = 1 (Work =1)
=> A = 1/X
B can finish the same work in Y
=> B * Y = 1 (Work =1)
=> B = 1/Y
When they both work together then their combined
...

---