Suppose 1 man does $\frac{1}{m}$ of the total work in 1 day.

It took 30 men working for 25 days and 35 men work for 11 days to complete the work, i.e.

$$30\times 25\times \frac{1}{m} + 35\times 11\times \frac{1}{m} = 1$$

$$m=1135$$

Suppose 30 men would’ve completed the job in x day,

i.e., $$30\times x\times \frac{1}{1135} = 1$$

$$x = \frac{1135}{30} = 37.833$$

So it would have took them nearly 38 days to complete the work on their own, and the delay would be 1 day.