The serial number on a bill consists of a letter followed by nine digits and then a letter. How many different serial numbers are possible, given the following conditions?

Question

(a) Letters and digits cannot be repeated.

(b) Letters and digits can be repeated.

Answer 1

a) 26*10*9*8*7*6*5*4*3*2*25

b) 26*10*10*10*10*10*10*10*10*10*26