This questions involves ratios, the job with 30 men can be done in 15 days, so the ratio from amount of men to amount of days the job will take to complete, it is 30:15. If we had 10 men instead of 30, 10 is one-third of 30, so you have to divide by 3 on each side of the ratio to get your answer.
If we do that we get the ratio 10:5.
Meaning that the 5 is the amount of days needed to be taken in order the complete the job.
