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How many ways can you find to put in operation signs (+, −, ×, ÷) to make 100?

Here we make the model more realistic, by exploring the effect of movement by a population on the spread of an infectious disease. There's an interactivity that allows you to party, commute, and visit friends.

Draw some stars and measure the angles at their points. Can you find and prove a result about their sum?

Factor track is not a race but a game of skill. The idea is to go round the track in as few moves as possible, keeping to the rules.

Can you find an efficient method to work out how many handshakes there would be if hundreds of people met?

Can you crack these cryptarithms?

How much do you have to turn these dials by in order to unlock the safes?

Can you describe the journey to each of the six places on these maps? How would you turn at each junction?

"Ip dip sky blue! Who's 'it'? It's you!" Where would you position yourself to be chosen?

This problem challenges you to find out how many odd numbers there are between pairs of numbers.