Find the number of natural numbers which are less than `2xx10^8` and which can be written by means of the digit 1 and 2.

How many different numbers which are smallerr than 2xx10^(8) and are divisible by 3, can be written by means of the digits 0,1 and 2?

Find the sum of all natural numbers less than 1000 which are neither divisible by 5 nor by 2.