(1/6+1/10+1/15):(3/5-1/3-1/4)
(1/6+1/10+1/15) : ( 3/5-1/3-1/4)= ( 5/30+3/30+2/30): ( 36/60-20/60-15/60)= 10/30 : 1/60= 10/30 х 60/1= 20