This is tricky. You need to switch between discount and no discount. But step by step, let's figure this out. First, how much does 3 tickets cost? It's 3 x $7.50, BUT WAIT! Got DISCOUNT of $5, so minus 5 from full price.
ANS: 17.50 for 3 tickets.
Now you must use guess and check method.
Since the amount Mr Faroek spent is 172.50, it looks very close to 17.50 x 10 or 175. SO I am guessing the answer is close to that. Let's see how many tickets can 175.00 buy.
175 / 17.50 = 10 sets of 3 tickets

But this is too many. So let's guess with 1 set less, which is 9 sets of 3 tickets.
175 - 17.50 = 157.50
So Mr Faroek spent 172.50, which means he bought at least 9 sets of 3 tickets, and then maybe one or two more tickets., but without discount for the remainder.

The amount he spent on undiscounted tickets is:
172.50 - 157.50 = $15

so from here, it is quite easy. $15 is the price of 2 x $7.50 tickets,

so he bought 9 sets of 3 (9 x 3 = 27) and 2 tickets, or 27+2 = 29 tickets.