We have two responses for you

Hi Farah,

Break up the problem into intermediate problems that can be solved step by step.

How many man-days does the work take? (Multiply the number of men needed by the number of days needed).

After 3 days, how many man-days have been spent on the work? How many man-days are left to be done?

If 3 more men join the work crew, how many man-days can the large crew apply to the work each day? How many days will it take at this rate to complete the job?

Cheers,
Stephen La Rocque.

Dear Farah,

your best bet would be to figure out how much work a single man does. Once you figure out how much work each man does, figure out how much work is done after 3 days. Then you should be able to figure out how much work it would take for 15 men to complete
the job.

In Summary:
1) Figure how much work (that is days of work) each man does for the job to be completed in 8 days
2) Calculate how much work has been done after 3 days, so you can figure out how much is left to do.
3) Calculate how long it would take 15 men to finish the job.

Hope this helps!

Brent