A can do $$\frac{3}{5}th$$ of a work in 12 days, B can do $$\frac{1}{3}rd$$ of that work in 15 days. They worked together for 12 days and then A left the work , B alone will complete the remaining work in?

Given A does (3/5)th of work in 12 days
so A takes 12*5/3 =20 days for complete work 
similarly B takes 15 days for (1/3)rd work and so for complete work it takes 45 days
LCM of 20 and 45 is 180 units
Each day 'A' does 180/20 =9 units of work
Each day 'B' does 180/45 = 4 units of work
So each day both can complete 13 units of work
In 12 days they complete 12*13=156 units of work
So 180-156-24 units
B can complete 24 units in 24/4 =6 days