Work done by x in a day=1/20
Work done by x in 4 days=4(1/20)=1/5
Work remaining=1-1/5=4/5
Work done by x and y in a day=(1/20 + 1/12)= 8/60=2/15
days:work done::days:work done
1:2/15::x:4/5
x=(4/5)(15/2)=6
Both of them completed the remaining work in 6 days.
Total number of days =4+6=10