The installation time is random, but on the average, it takes 25 sec to install one file, with a standard deviation of 4 sec.?

It takes 25 sec to install one file, with a standard deviation of 4 sec
To install 49 files, it takes (49)(25) = 1225 sec (20.4167 minutes) with a standard deviation o 49(4) = 196 sec (3.2667 min)
Assuming normal distribution:
P( x < 20) = ?

μ = 20.4167
σ = 3.2667
standardize x to z = (x - μ) / σ
P(x < 20) = P( z < (20-20.4167) / 3.2667)
= P(z < -0.1276) = 0.4483
(From normal probability table)

The total time is distributed with mean 20.417 minutes and standard deviation 0.58 seconds, or 0.0096 minutes (not 3.2667 minutes). You would need to be 43 standard deviations below the mean to be at 20 minutes. But at 3 or 4 standard deviation, the assumption of normality breaks down.

So although the math says the probability is practically zero, the real world say you *might* get a few occasions where it takes less than 20 minutes.

I don't know