Explaining, convincing and proving

In a league of 5 football teams which play in a round robin tournament show that it is possible for all five teams to be league leaders.

Blue Flibbins are so jealous of their red partners that they will not leave them on their own with any other bue Flibbin. What is the quickest way of getting the five pairs of Flibbins safely to the new planet?

Can you place the nine cards onto a 3×3 grid such that every row, column and diagonal has a product of 1?

If Clara spends £23 on books and magazines, how many of each does she buy?

A little bit of algebra explains this 'magic'. Ask a friend to pick 3 consecutive numbers and to tell you a multiple of 3. Then ask them to add the four numbers and multiply by 67, and to tell you the last two digits of her answer. Now you can really amaze her by giving the whole answer and the three consecutive numbers used at the start.

You can work out the number someone else is thinking of as follows. Ask a friend to think of any natural number less than 100. Then ask them to tell you the remainders when this number is divided by 3, 5 and by 7...