Egyptian heritage

Let N N be a positive multiple of 7, that leaves a remainder of 1 when divided by 2, 3, 4, 5 or 6.

Fortunately, there are infinite values of N N .

Find the sum of the least 10 values of N N .

3130 3010 121 21910

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2 solutions

Mark Hennings
Jul 27, 2019

The LCM of 2,3,4,5,6 is 60. Thus we want to find integers N N such that N 0 ( m o d 7 ) N \equiv 0 \pmod{7} while N 1 ( m o d 60 ) N \equiv 1 \pmod{60} . Thus N = 7 a = 60 b + 1 N = 7a = 60b + 1 , so that 7 a 60 b = 1 = 2 × 60 17 × 7 7 ( a + 17 ) = 60 ( b + 2 ) \begin{aligned} 7a - 60b & = \; 1 \; = \; 2\times 60 - 17 \times 7 \\ 7(a + 17) & = \; 60(b + 2) \end{aligned} so that a + 17 = 60 c a + 17 = 60c and b + 2 = 7 c b+2 = 7c for some integer c c , and hence we deduce that N = 420 c 119 c Z N \; = \; 420c - 119 \hspace{2cm} c \in \mathbb{Z} The sum of the 10 smallest positive such integers is c = 1 10 ( 420 c 119 ) = 21910 \sum_{c=1}^{10}(420c - 119) \; = \; \boxed{21910}

A true genius! I don't know how you solve these questions.

A Former Brilliant Member - 1 year, 10 months ago

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You might want to read about Diophantine equations. In particular for this problem, look up the Chinese Remainder Theorem.

Mark Hennings - 1 year, 10 months ago

Using the same deduction as Mark Henning re 420, but not realizing how to get the 301 number, I just searched for it. 1 2 ( 9 10 ) 420 + 10 301 = 21910 \frac{1}{2} (9\ 10) 420+10\ 301=21910 . That is, ten copies of 301 plus 9 ( 9 + 1 ) 2 \frac{9(9+1)}{2} copies of 420. The fraction is the 9 t h 9^{th} triangular number.

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