The median is defined to the value at which there is an equal number of observations less than and greater than it. For example, the median of: 1,2,3,4,5 is 3 because there are 2 values less than 3 and 2 values greater than 3.

Say you have ten cats. So 3 of them weigh 2-kg and 8 weight 5-kg. What is the weight at which there will be an equal number of cats lighter and heavier than it?