Deepthi Subbu wrote:A bowl contains 50 marbles. What is the probability that a blue or red marble is selected ?

1. The probability that a blue marble is selected is 3/10
2. There are 20 red marbles in the bowl

Say the number of blue marbles = B and number of red marbles = R
Hence, required probability = (B + R)/50 = (B/50) + (R/50)

(B/50) = Probability of picking a blue marble
(R/50) = Probability of picking a red marble

Statement 1: The probability that a blue marble is selected is 3/10
Hence, (B/50) = 3/10
But we don't about the red ones.

Not sufficient

Statement 2: There are 20 red marbles in the bowl
Hence, (R/50) = 2/5
But we don't know about the blue ones.

Not sufficient

1 & 2 Together: We have both the information.
Required probability = (3/10) + (2/5) = 7/10

Sufficient.

The correct answer is C.

Note: We don't know whether there are other colored marbles or not. Thus we can't assume there are only red and blue marbles.