ANS:B - 13
Let the required number of days be x.
Less persons, More days (Indirect Proportion)
More working hours per day, Less days (Indirect Proportion)
Persons |
30 |
: |
39 |
 |
:: 12 : x |
Working hours/day |
6 |
: |
5 |

30 x 6 x
x = 39 x 5 x 12
x = |
(39 x 5 x 12) |
(30 x 6) |
x = 13.