Find the probability that one ball is white when two balls are drawn at random from a basket that contains 9 red, 7 white and 4 black balls.

Total number of elementary events = $20C2$ ways =190

There are 7 white balls out of which one white can be drawn in 7 $C1$ ways and one ball from remaining 13 balls can be drawn in $13C1$ ways.

So,favourable events =  $7C1*13C1$

Therfore,required probability = ( $7C1*13C1$)/ $20C2$=91/90