**CONCEPT:-**Time and Work

-Total work is a sum of the individual works and the collective work(Jay and Anup here)**SOLUTION:** Let the total work be LC(15,30)=30 units.

Efficiency of Jay is 1units/day and that of Anup is 2 units/day.

Together, the efficiency is (1+2)=3 units/day

Let Jay worked for "x" days alone.

Then, 1*(x) X (18-x)*3=30

=>x=12 days (C)