1) What is the probability that the last three digits of a randomly selected phone number are all prime? Express your answer as a common fraction
2) The sum of the digits of a number is 9. What is the probability that the number is prime?
1) What is the probability that the last three digits of a randomly selected phone number are all prime? Express your answer as a common fraction
2) The sum of the digits of a number is 9. What is the probability that the number is prime?
1) There are four prime digits: $\displaystyle 2,~3,~5,~\&~7$.
Picking a digit at random there is $\displaystyle \frac{4}{10}$ probability that it is prime.
Now you finish.
2) If the sum of the digits is multiple of nine then the number is divisible by nine.
What does tell you?