(1/10+1/12+1/15)*(6-x)+(1/12+1/15)*x=1
1/4*(6-x)+3/20*x=1
3/2-1/4*x+3/20*x=1
(1/4-3/20)x=3/2-1
1/10*x=1/2
x=1/2÷1/10
x=5