Question

Martha types a 4−digit code into a keypad to unlock her car doors. The code uses the numbers 1 and 0. If the digits are selected at random, what is the probability of getting a code with exactly two 1's? Enter your answer as a simplified fraction.

The probability of getting two 1's is_________.

Answer 1

3/8

Explanation:

1. We list all codes that start with 0 and have 0 as second digit:

2. We list all codes that start with 0 and have 1 as second digit:

3. We list all codes that start with 1 and have 0 as second digit:

4. We list all codes that start with 1 and have 1 as second digit:

5. We find the number of codes which have exactly 2 zeros: 6 codes

6. The total number of outcomes is: 16 codes

7. We find the probability that the number of codes has exactly 2 zeros: 6/16 3/8

Answer 2

The probability of getting two 1's is 3/8

Explanation:

Martha types a 4−digit code into a keypad to unlock her car doors.

The code uses the numbers 1 and 0

So the sample space of all the possible combinations is given by

1. 0 0 0 0

2. 0 0 0 1

3. 0 0 1 0

4. 0 0 1 1

5. 0 1 0 0

6. 0 1 0 1

7. 0 1 1 0

8. 0 1 1 1

9. 1 0 0 0

10. 1 0 0 1

11. 1 0 1 0

12. 1 0 1 1

13. 1 1 0 0

14. 1 1 0 1

15. 1 1 1 0

16. 1 1 1 1

So there are total 16 possible outcomes.

Now count the number of combinations that has exactly two 1's

4. 0 0 1 1

6. 0 1 0 1

7. 0 1 1 0

10. 1 0 0 1

11. 1 0 1 0

13. 1 1 0 0

So there are 6 possible outcomes with exactly two 1's

The required probability is

P(two 1's) = Number of outcomes with two 1's/Total number of outcomes

P(two 1's) = 6/16

P(two 1's) = 3/8

Therefore, the probability of getting two 1's is 3/8