A can do 40% of a work in 12 days, whereas B can do 60% of the same work in 15 days. Both work together for 10 days. C completes the remaining work alone in 4 days. A, B and C together will complete 28% of the same work in:

Time taken by A to complete whole work = $$\frac{12}{40} \times$$ 100 = 30 days
Time taken by B to complete whole work = $$\frac{15}{60} \times$$ 100 = 25 days
Let the total work be 150 units.
($$\because$$ LCM of 30 and 25 is 150.)
Efficiency of A = 150/30 = 5
Efficiency of B = 150/25 = 6
Work done by A and B in 10 days = (5 + 6) $$\times$$ 10 = 110
Remaining work = 150 -110 = 40
Efficiency of C = 40/4 = 10
28% of the work = 150 $$\times \frac{28}{100} = 42
Time taken to complete 28% of the work by A, B and C = $$\frac{42}{5 + 6 + 10}$$ = 42/21 = 2 days