In a bus, there are 20 passengers, and each has many coins of 10, 15, and 20 kopecks. Each passenger needs to pay 5 kopecks.

Can they do this using (including exchanging among themselves) a) 24 coins; b) 25 coins?
a) Let it be so. Each passenger received change, which means that out of 24 coins, no more than four, or 80 kopecks, ended up in the cash register. However, the total fare is one ruble.

b) It is sufficient to prove that four passengers can pay using five coins. The first puts a 20-kopeck coin in the cash register, the second gives two 10-kopeck coins to the third and fourth, and they each give 15 kopecks to the first and second.

## Answer

a) They cannot; b) they can.