Why do this problem?
is a good everyday example of the use of addition and multiplication. It also helps pupils see the use of arithmetic in practical situations.
Start off a general discuss about posting to see what they know about posting letters and cards. When they are happy to talk about stamps present them with the challenge. You may prefer to update the charges and adapt the challenge to suit the current pricing system.
Tell me how you have worked this out.
Pupils could invent their own question based upon the present-day costs of postage.