Question

An identity thief watches someone enter a 4-digit PIN and manages to see the first two digits. If the thief tries guessing the PIN, how many possibilities would there be?
a.
81
b.
90
c.
100
d.
200

Answer 1

Think of it this way: your friend is thinking of a year, and you have to guess which.  She has told you the year is in the 20th century (1900s).
There are 100 years in the 20th century (1901-1999 plus 1900).
There can be 100 possibilities

Answer 2

100 combinations can be formed from the last two digits.  So the answer is (c).