An IQ test consists of 80 multiple choice questions. A question is selected at random. Find the probability that the question number contains exactly one '7' ( is the answer 1/5 ? )
No, the answer is not 1/5.
To find the probability that the question number contains exactly one '7', we need to determine the total number of questions that contain exactly one '7' and divide it by the total number of questions.
There are ten possible digits (0-9) for the question number. To have exactly one '7', there are two possible scenarios:
1. The question number is in the form of '7X' where X is any digit from 0-9 except 7.
2. The question number is in the form of 'X7' where X is any digit from 0-9 except 7.
For scenario 1, there are 9 choices for the digit X (0-9 excluding 7), and for scenario 2, there are also 9 choices for the digit X. So, there are a total of 9 + 9 = 18 question numbers that contain exactly one '7'.
Thus, the probability that the question number contains exactly one '7' is 18/80, which simplifies to 9/40. So, the answer is 9/40.