Sam has forgotten his friend’s seven-digit telephone number. He remembers the following: the first three digits are either $635$ or $674,$ the number is odd, and the number nine appears once. If Sam were to use a trial and error process to reach his friend, what is the minimum number of trials he has to make before he can be certain to succeed?
635_ _ _ _
674_ _ _ _
The total no. of Trial and Error process with 635 as prefix is (324 + 324 + 324 + 729) = 1701.
This 1701 combinations will repeat with 674 as prefix also.
Minimum no. of trials = 1701 + 1701
= 3402 (option 3)