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At how many times between 10 and 11 o'clock are all six digits on a digital clock different?

The teacher has forgotten which pupil won which medal. In how many different ways could he give the medals out to the pupils?

Two of the four small triangles are to be painted black. In how many ways can this be done?

How many different phone numbers are there starting with a 3 and with at most two different digits?

In how many different ways can a row of five "on/off" switches be set so that no two adjacent switches are in the "off" position?

Can you crack these very difficult challenge ciphers? How might you systematise the cracking of unknown ciphers?

Can the pdfs and cdfs of an exponential distribution intersect?

How many routes are there in this diagram from S to T?