Suppose someone is going to read out to you the numbers 1-100 in random order, but they will miss one out. When they have finished you will be asked which number was missed.

You have no means of recording the numbers as they are read, and although you are quite good at mental arithmetic you only have an average memory and remembering the entire list of numbers as they were read out is beyond you.

What is the best way to identify the missing number?