One of the teachers at a school is chosen at random. The probability that this teacher is female is 3/5. There are 36 male teachers at the school. Work out the total number of teachers at the school.

Assuming that every teacher at the school is either male or female, we can work out that the probability that the teacher chosen at random is male is 2/5, because 1 - 3/5 = 2/5, and therefore we know that two fifths of the total number of teachers at the school are male. The question tells us that there are 36 male teachers at the school, and from this we know that 36 is 2/5 of the total number of teachers at the school.
Now, all we have to do is convert the value that represents 2/5 of the total number of teachers (36) to 5/5 (or 1). We can do this by dividing 36 by 2 to get 18, which represents 1/5 of the total number of teachers, and then we can just multiply 18 by 5 to get 90, which is our answer.