A box contains 4 red, 5 green and 6 white balls. A ball is drawn at random from the box. What is the probability that the ball drawn is either red or green ?

Answer: Option B

Total number of balls = (4 + 5 + 6) = 15
P (drawing a red ball or a green ball) = P (red) + P (green)
$$\begin{array}{rl}& =\left(\frac{4}{15}+\frac{5}{15}\right)\\ & =\frac{9}{15}\\ & =\frac{3}{5}\end{array}$$