17) B

Total number of children = 1 + 9 = 10

She had enough money left to buy 10 children a packed of bubble gum each.
Amount of money left = 0.25 x 10 = £2.50

So total money spent = 125 – 2.5 = £122.5

Two chose to go on the Apocalypse. Since they chose Apocalypse, they must have gone on the Carousel.
Amount spent on these two kids = 2 (9 + 3.5) = 2 (12.5) = £25

Money left = 122.5 – 25 = £97.5

All the other kids must have chosen the Armageddon as their main ride.
Let number of kids that chose Armageddon + Dodgems = d
Let number of kids that chose Armageddon + Helter Skelter = h

Amount spent by d = d (7.5 + 5) = 12.5d
Amount spent by h = h (7.5 + 4.5) = 12h

We know that two people went on the Apocalypse. Therefore all the other 8 kids must have chosen the Armageddon.
So, d + h = 8

12.5d + 12h = 97.5
d + h = 8

Using simultaneous equations, we get

12.5d + 12h = 97.5
12d + 12h = 96

0.5d = 1.5
d = 3

d = number of children that chose Dodgems + Armageddon
Hence, the answer is 3. Option B is correct.