Answer: £450
Explanation:
If they bought 2 tickets, they could get an extra ticket for free. As there are three of them, they would buy 2 tickets, get one for free and then buy another ticket to make it four.
In that case they would have only paid for 3 tickets.
The total cost of 3 tickets is:
= 600 * 3
= £1,800
The 4 of them would therefore have to pay:
= 1,800 / 4
= £450