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)