Question

Assume every 10-digit natural number is a possible telephone number except those that begin with 0, 1 or 2. What fraction of telephone numbers begins with 8 and ends with 8?

Answer 1

Answer: 1/70

Explanation:

This is a question that can also be interpreted as what is the probability of having the first number of a phone number to be 8 and the last number of the phone number to also be 8. This answer gives the fraction of the phone numbers that starts with 8 and end with 8.

Since three numbers (0,1,2) cannot start a phone number and we are left to pick from 7 numbers,

then the probability of figure "8" starting phone number = 1/7

Since all 10 numbers can possibly end a phone number,

then the probability of having figure "8" as the last digit of a phone number = 1/10

Hence probability of having "8" as the first and last digit of a phone number = fraction of total telephone numbers that begin with digit 8 and end with digit 8 = 1/7 × 1/10 = 1/70.