What is the median, upper quartile and lower quatile. How do we find them?

The easiest way to explain this would be to use an examplar question. For example:

Ben is given a sequence of numbers and is told to find the median, lower quartile and upper quartile.

1,4,5,6,8,9,11,12,14,15,16,17

The median is the number in the sequence (has to be rearranged to increasing numbers as shown in the question) which is in th MIDDLE of the sequence. In this case it would be 11.

The lower quartile is the median in between the smallest number and the median i.e. in between 1 and 11, which will be 6.

For the upper quartile, you find the median between the median and the largest number i.e. between 11 and 17, which will be 15 according to the question above.