Final answer:
To find the probability of getting a red ball, subtract the sum of the probabilities of getting a white and black ball from 1. The probability of getting a red ball is 1/2. If the bag contains 20 black balls, the total number of balls in the bag is 50. The correct answer is d. 1/2.
Step-by-step explanation:
To find the probability of getting a red ball, you can subtract the sum of the probabilities of getting a white ball and a black ball from 1. Since the probability of getting a white ball is 3/10 and the probability of getting a black ball is 2/5, the probability of getting a red ball is 1 - (3/10 + 2/5) = 1/2.
If the bag contains 20 black balls, then we can find the total number of balls in the bag using the equation:
Probability of a white ball + Probability of a black ball + Probability of a red ball = 1
3/10 + 2/5 + Probability of a red ball = 1
Probability of a red ball = 1 - (3/10 + 2/5)
Probability of a red ball = 1/2
Let's represent the total number of balls in the bag as x. Since the probability of a black ball is 2/5 and there are 20 black balls, we can set up the proportion:
2/5 = 20/x
Cross-multiplying, we get:
2x = 5 * 20
2x = 100
x = 50
Therefore, the total number of balls in the bag is 50.