You are close!
Let's set up a proportion, where x is the number of men doing the job in 15 days, and y is the number of days 10 men need to do the job.
\(\frac{x}{15} = \frac{10}{y}\)
Cross multiplying will give us xy = 150.
Since we need to find y, we now need to find x in terms of y. This will give us x = 150/y.
Now, our new proportion is
\(\frac{\frac{150}{y}}{15} = \frac{10}{y}\)
Now, we can either form a system of equations, OR, we can directly see that when y is 15, all the euqations work out.
Therefore, you will need 15 days for 10 men to do the job.