Question

How many 4 digit PINs have two 1s but you don’t know where?

Answer 1

Answer:

Given that,

How many 4 digit PINs have two 1s but you don’t know where?:

Since a pin is made of digits 0 to 9, A pin with two 1's but unknown locations can be made in

The 2 locations where 1 will be placed is chosen in ways,in the other 2 locations we have to take 1 choice from 10 choices of numbers (0-9) each , in 10 10=100 ways.

Note, here my pin may start with 0 because ATM pins do start with 0 and also nowhere it is mentioned that 0 cannot be first digit.

Hence a 4 digit ATM pin with two 1's can be chosen in 600 ways.