There are 10 balls in a bag. The balls are the same apart from their colour. Of the 10 balls, 3 are black, 5 white, 2 red. What is the probability of picking a white ball from the bag.

To calculate the probability assume it is equally likely to choose any of the 10 balls. This is reasonable as we are told that the balls are the same apart from their colours. If we label each ball by numbers so 1 to 3 are the black one, 4 to 8 the white ones, 9 and 10 the red ones, then we are equally likely to choose any number from 1 to 10. However, in terms of the colour of the balls, there are more balls with the colour white, the numbers 4, 5, 6, 7, 8 are all white balls, compared with numbers 1,2,3 being black, and numbers 9, 10 being red. The probability of picking a white ball is then given by the number of white balls over the total number of balls. So P(white) = 5 / 10 = 1/2.