Probability-2

Another way to look at it is to realize that the order in which they are taken out is irrelevant since no information is revealed about the first three.

Therefore, the probability is the same as it would be if it were the first one taken out, which is $\dfrac{4}{10} = \boxed{\dfrac{2}{5}}$

There are 4 ways to pick which colored balls are the three taken out first.

We can have www, wwb, wbb, and bbb.

The probability of www is 4/10 * 3/9 * 2/8, and then a 1/7 chance for the fourth being white, leaving 4/10 * 3/9 * 2/8 * 1/7 = 1/210.

The probability of wwb is 4/10 * 3/9 * 6/8, and then a 2/7 chance for the fourth being white, leaving 4/10 * 3/9 * 6/8 * 2/7 = 3/35.

The probability of wbb is 4/10 * 6/9 * 5/8, and then a 3/7 chance for the fourth being white, leaving 4/10 * 6/9 * 5/8 * 3/7 = 3/14.

The probability of bbb is 6/10 * 5/9 * 4/8, and then a 4/7 chance for the fourth being white, leaving 6/10 * 5/9 * 4/8 * 4/7 = 2/21.

1/210+3/35+3/14+2/21=2/5.