This is an example of "classical probability". Whenever the probability of all events is the same, then the probability is calculated by dividing the number of "favorable events" of total events.
For this problem, the total number of events is the total number of balls, it is 40. The number of favorable events it the number of pink balls, it is 6.
So, the answer is 6/40, which can be written as 3/20.
