10 exam has four difficult to sign, and three people draw lots. A draws first, B draws again, C Finally, find the probability of the following events.

Not only is it the first step of AC, but the probability that A can't draw out the difficult sign is 6/10;

In the second step, the probability that B draws a simple ticket is 4/( 10- 1).

So the probability of event A is 6/ 10*4/9=4/ 15.