In the question it is asked how many machines to be added so Isn’t A equals 24 ?

Yes. 32-8 = 24

Blockquote

| Machines = 8 | Time(sec) = 20 | Bolts = 250 |

| Machines = 8 | Time(sec) = 10 | Bolts = 125 |

[ In half the time (10 secs) they will produce half amount of bolts ]

| Machines = 1 | Time(sec) = 10 | Bolts = 125/8 |

The final equation will be (125/8)*X = 500 [X = Number of machines]

You answer should be 32.