" Number of different natural numbers which are smaller than two hundred million & using only the digits I or "2" is "

Number of different natural numbers which are smaller than two hundred million & using only the digits 1 or 2 is :

Is the difference of two natural numbers always smaller than the given numbers?

Numbers of natural numbers smaller than ten thousand and divisible by 4 using the digits 0,1,2,3 and5 without repetition is n then

Numbers of natural numbers smaller than ten thousand and divisible by 4 using the digits 0,1,2,3 and5 without repetition is n then

The HCF of two numbers can be smaller than the smaller number ?