HARD
IOQM - PRMO and RMO
IMPORTANT
Earn 100

A natural number k>1 is called good if there exist natural numbers a1<a2<<ak such that 1a1+1a2++1ak=1. Let fn be the sum of the first n good numbers, n1. Find the sum of all values of n for which fn+5fn is an integer.

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Important Questions on Sequence and Series

EASY
IOQM - PRMO and RMO
IMPORTANT
Consider the set E of all positive integers n such that when divided by 9,10,11, respectively, the remainders (in that order) are all >1 and form a non-constant geometric progression. If N is the largest element of E, then find N4.
MEDIUM
IOQM - PRMO and RMO
IMPORTANT
Let N=6+66+666.++66666, where there are hundred 6 's in the last term in the sum. How many times does the digit 7 occur in the number N ?