How many different positive numbers smaller than 2∗10^8 can be formed using the digits 1 and 2 only?  Choices:
A) 256
B) 510
C) 512
D) 766
E) 6561
Any combination of number starts with digit "2" is greater than 2*10^8   Total possible combinations with 1 and 2 = 2^9   Total possible combinations with 2 as starting digit = 2^8   Numbers smaller than 2*10^8 can be formed = 2^9 - 2^8   = 2^8(2-1)   = 256.   256nswer : 256
<result>A</result>
How many different positive numbers smaller than 2∗10^8 can be formed using the digits 1 and 2 only?