Explaining, convincing and proving

Jack does a 20-question quiz. How many questions didn't he attempt?

Fill in the grid with A-E like a Sudoku. Which letter is in the starred square?

Can you visualise whether these nets fold up into 3D shapes? Watch the videos each time to see if you were correct.

Mr Ross tells truths or lies depending on the day of the week. Can you catch him out?

Weekly Problem 21 - 2010
How many diagonals can you draw on this square...

The squares of this grid contain one of the letters P, Q, R and S. Can you complete this square so that touching squares do not contain the same letter? How many possibilities are there?

Sarah's average speed for a journey was 2 mph, and her return average speed was 4 mph. What is her average speed for the whole journey?

What is the smallest number of jelly babies Tom must take, to be certain that he gets at least one of each flavour?

Weekly Problem 32 - 2015
Can you work out the missing numbers in this multiplication magic square?