note that each stamp has a 9 digit identifying number on the bottom, like this one, 365293051 a. If any digit can be used in any position, h

a)  We could have any of 10 digits in any of the nine positions......so, the total number of different identifiers  = 10⁹ = one billion

b) I don't understand this one.......could you be a little clearer ???

For part a:

There are 9 positions in the identifying number and each position can hold any one of ten digits, so there are 

   10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10    or     10^9   =   1,000,000, 000 possibilities.

For part b:

I have no idea what the 120 stamps refer to; perhaps someone else can answer that ...