Answer and commentary

Since we are finding an approximation, the exact answer cannot matter.
This would be an easy question to find the exact answer to using a calculator, which the builder could get, if that was important. But we want a quick method to get a feel for the answer.
Normally we would work to 1 significant figure. That means only using the first digit, but keeping to the correct size.
47.9 is close to 50.
8.94 is close to 9 but 10 would be easier and close enough.
11.7 is close to 12 but again 10 is fine because we went up a bit with the 8.94, so going down a bit with the 11.7 will compensate.
39.4 is close to 40.
So, our question becomes:

Now is the same as .
So, we have
which is 25.

This explanation is long, but look again at the calculation and see how you could have done it in your head in one go.
Answer: 25