AlgebraDifficulty 3.7AMC 10/12Find the answerEstonia
How many positive integers, where the only allowed digits are 0 and 1, are less than 1111100100?
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Solutions — 2
Solution 1
The given number has 11 digits. There are 211−1=2047 positive integers with at most 11 digits each of which is either 0 or 1. We solve the problem by subtracting the number of positive integers that are not less than the given number. The 11-digit numbers larger than the given number are all of the form 11111abcde. There are 25=32 numbers in this form. Among them, 4 numbers 1111100000, 1111100001, 1111100010 and 1111100011 are less than the given number. Thus the number of positive integers consisting of zeros and ones and being less than 1111100100 is 2047−32+4=2019.
Solution 2
Ordering any two digit sequences that constitute a positional representation on different bases does not depend on the base. This means that if a number is larger than another number on base 2 then the first number is larger than the second number also on base 10. The number 1111100100 on base 2 equals 210+29+28+27+26+25+22=2020 on base 10. Hence, to solve the problem, it suffices to count all positive integers less than 2020. The result is obviously 2019.
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Source: MathNet,
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