Mr and Mrs Phelps and their three children go on a train journey.

Mr Phelps pay for 2 adult tickets and 3 children's tickets.

The price of an adult ticket is $8.40.

The price of a children’s ticket is half the price of an adult ticket.

Mr Phelps pay for the tickets with three ten-dollar notes.

The cashier only has 20-cent coins in the till.