In the box we have --> Red--> 2 Black--> 3 Blue--> 4
Now we are picking up 2 blue balls at random which can be done in (4C2 / 9C2) ways
after that we are picking up a black ball, number of ways to do that --> (3C1 / 7C1) ways
so we are having probability --> (4C2 / 9C2) x (3C1 / 7C1)
which gives us--> 0.07142
As the ans is asked in % so, --> 0.07142 * 100 = 7.14 %