Question

How many 60 W lightbulbs can be hooked in parallel to 120 V without blowing a 10 A fuse? (see Fig. 25- 20 for a picture of the circuit.)

Answer 1

60/120 = 0.5 amps

10/0.5 = 20, so you need 21 to exceed 20 amps.

In theory.

But incandescent lamps have a high current surge when they turn on. If you turn on all 20 at once, you will get a 40 to 100 amp surge.
And the amount of current is not know very accurately, it could be ±20%

And 20 amp breakers presented with 21 amps will take a long time to pop, perhaps hours, depending on exact type of breaker.