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Earn 100

Let S be the set of positive integers n for which 1n has the repeating decimal representation 0.ab¯=0.ababab…. , with a and b as different digits. What is the sum of the elements of S?

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Important Questions on Numbers and Their Properties

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If a, a+2, and a+4 are prime numbers, then the number of possible solutions for a is:
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Suppose N is an integer such that the sum of the digits of N is 2, and 109<N<1010. How many values of N are possible?
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Ten students solved a total of 35 questions in a Maths Olympiad. Each question was solved by exactly one student. There is at least one student who solved exactly one problem, at least one student who solved exactly two problems, and at least one student who solved exactly three problems. What is the minimum number of students who has/have solved at least five problems?
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N! has 37 zeroes at its end. How many values of N is/are possible?
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In the abovementioned question, how many values of N will be odd?
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N! is having 30 zeroes at its end. How many values of N is/are possible?
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What is the remainder when 11+22+33+…+100100 is divided by 4?
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An intelligence agency forms a code of two distinct digits selected from 0,1,2,...9, such that the first digit of the code is not zero. The code, handwritten on a slip, can however potentially create confusion when read upside down - for example, the code 91 may appear as 16. How many codes are there for which no such confusion can arise?