Hi,
I have a question on probability. Here is the question
John wrote a phone number and can remember that the number had 7 digits, the digit 1 appeared in last three places and 0 did not appear at all. What is the prob that phone number contains at least 2 prime digits?

Ans: 1 MINUS ( prob of no prime digits + having 1 prime digit)

4 prime digits ( 2,3, 5,7)

4 non prime digits ( 4,6,8,9)
prob of choosing a prime digit = 1/2

4 options of one prime digit
one option of no prime digit with a prob of ( 1/2) to the 4 th power

1 MINUS ( 1/2) to the 4th power + 4*(1/2) to the 4th power

Since there are 4 choices for nonprime, why doesn't this part get multiplied by 4 like one prime digit prob?

Thanks