There were 3 printers in the workroom. One was very slow, one was faster, and one was very fast. They were nicknamed Sloth, Monkey, and Cheetah. Ms. Morrison needed to print a large number of pages, and she knew that if she could use two of the printers, she could finish the job in 45, 60, or 90 minutes, depending on which two printers were available. What if she could use all 3 at the same time? How many minutes would it take to finish printing?
Cheetah does C pages/min;
Monkey does M pages/min;
Sloth does S pages/min.
Total number of pages = P
(C + M)*45 = P
(C + S)*60 = P
(M + S)*90 = P
Solve these for C, M and S in terms of P
C = P/72; M = P/120; S = P/360
So (C + M + S)*time = P or (P/72 + P/120 + P/360)*time = P
I'll let you finish.