I think I figured it out but here is the problem

One person can do a task in 10hrs, a second can do it in 15 and the third can do it in 20. Together how long will it tak

so here is what I came up with

x/10+x/15+x/20=1

60(x/10+x/15+x/20)=1*60

6x+4x+3x=60

13x=60

13x/13=60/13

x=4.62