Maths Help Please

Ok, so I can't work out the answer to this question for my maths homework and I need some help please! So here it is:

It is £1.50 to park a car in a car park.
This can be paid in £1 coins or 50p coins.

In one day, the car park collected 136 £1 coins and 208 50p coins.

How many drivers paid with three 50p coins?

Well, you know that there are two ways to pay: a £1 coin and a 50p coin, or three 50p coins. Therefore, the people who put £1 coins in must also have put one 50p piece in each, which counts for 136 of the 50p coins.

If you do 208 - 136 you get 72, so you know that there are 72 50p coins which weren't put in by people paying with £1 coins, so they must have been put in by people paying with three 50p coins.

72 divided by 3 is 24, and that's your answer.

Hope this explained it, and that your homework deadline hasn't passed yet.