UPSC NDA Math Model Paper 8

There are a total of 7 red + 4 blue = 11 balls.
Probability of drawing 1 red ball =

‌7C1

‌11C1

=

7

11

.
Probability of drawing 1 blue ball =

‌4C1

‌11C1

=

4

11

.
Probability of drawing (1 red )AND(1 blue ) ball =

7

11

×

4

11

=

28

121

.
Similarly, Probability of drawing (1 blue )AND(1 red ) ball =

4

11

×

7

11

=

28

121

.

Probability of getting the balls of different colors =

28

121

+

28

121

=

56

121