The answer depends on the units used for 39.51: It should be blindingly obvious to anybody that the answer for 39.51 hours and 39.51 milliseconds will be very different.
4000
The number 3951 is already in its simplest form as it is an integer and cannot be simplified further. If you are asking for its prime factorization, 3951 can be factored into (3 \times 3 \times 439) or (3^2 \times 439). However, as a single whole number, 3951 does not simplify any further.
36 seconds
40
40
36 seconds
36 seconds
SIC 3951 covers makers of Pens, Mechanical Pencils, and Parts
40.00
If that's seconds, it rounds to 40.
Establishments primarily engaged in manufacturing mechanical pencils are classified in SIC 3951
It is closer to 36 than it is to 35.