Concept: The question tests on the basics of Rates(Work and Time)
Solution:
If C alone can finish a work in 60 days, then work done in 1 day = 1/60
If C does half the work as A and B in 1 day, then the work done by A and B in 1 day = 2 * 1/60 = 1/30
Working together, the total work done in a day = 1/30 + 1/60 = 3/60 = 1/20
Time taken to finish the work together (flipping numerator and denominator) = 20 days
Option C