(1/10+1/12+1/15)*(6-x)+(1/12+1/15)*x=1
(6-x)/10 +(1/12+1/15)*6-(1/12+1/15)*x+(1/12+1/15)*x=1
(6-x)/10+6*(4+5)/60=1
6-x+9=10
x=5