There are 10 discs initially, and the probability of drawing a black disc first is 3/10, leaving 9 discs, 7 of which are white. Probability of drawing a white disc next is 7/9, leaving 8 discs, 6 of which are white. Probability of drawing another white disc is 6/8.
Multiplying the probabilities we get: 3/10*7/9*6/8=7/40=0.175.