A bag contains 7 green and 8 white balls. If two balls are drawn simultaneously, the probability that both are of the same colour is -

Answer: Option D

Drawing two balls of same color from seven green balls can be done in $${}^7{C}_2$$ ways.
Similarly from eight white balls two can be drawn in $${}^8{C}_2$$ ways.
$$\begin{array}{rl}& P=\frac{{}^7{C}_2}{{}^{15}{C}_2}+\frac{{}^8{C}_2}{{}^{15}{C}_2}\\ & =\frac{7}{15}\end{array}$$