It's right.
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