Answer:
4
Step-by-step explanation:
A carpenter uses 4 nails to install each shelve
Check the picture below.
keeping in mind that the point of tangency for a radius line and a tangent is alway a right-angle, since the "red" chord is parallel to the "green" tangent line outside, then the chord is cutting the "green" radius there in two equal halves at a right-angle, as you see in the picture.
we know the chord is 10 units long, so 5 + 5, since is perpendicularity with the radius will also cut the chord in two equal halves.
anyhow, all that said, we end up with triangle you see on the right-hand-side, and then we can just use the pythagorean theorem.

28.26, because 6/2=3 3x3=9 3.14x9= 28.26
11:32 pm + 5 hours= 4:32pm
4:32 pm + 45 minutes = 5:17 pm
3/4 hour = 45 min