Consecutive!

I'm thinking of 8 consecutive positive integers. I arrange them in some order and find the following:

\hspace{5mm} \tiny\bullet \hspace{1.5mm} The 1 st 1^\text{st} number is a multiple of 2.
\hspace{5mm} \tiny\bullet \hspace{1.5mm} The 2 nd 2^\text{nd} number is a multiple of 3.
\hspace{5mm} \tiny\bullet \hspace{1.5mm} The 3 rd 3^\text{rd} number is a multiple of 5.
\hspace{5mm} \tiny\bullet \hspace{1.5mm} The 4 th 4^\text{th} number is a multiple of 7.
\hspace{5mm} \tiny\bullet \hspace{1.5mm} The 5 th 5^\text{th} number is a multiple of 11.
\hspace{5mm} \tiny\bullet \hspace{1.5mm} The 6 th 6^\text{th} number is a multiple of 13.
\hspace{5mm} \tiny\bullet \hspace{1.5mm} The 7 th 7^\text{th} number is a multiple of 17.
\hspace{5mm} \tiny\bullet \hspace{1.5mm} The 8 th 8^\text{th} number is a multiple of 19.

Find the smallest possible sum of the 8 consecutive integers I am thinking of.


The answer is 2732.

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2 solutions

Mark Hennings
May 15, 2018

If n n is the smallest number, then one each of n , n + 1 , n + 2 , n + 3 , n + 4 , n + 5 , n + 6 , n + 7 n,n+1,n+2,n+3,n+4,n+5,n+6,n+7 must be divisible by 2 , 3 , 5 , 7 , 11 , 13 , 17 , 19 2,3,5,7,11,13,17,19 . Thus, in particular, two of these numbers must be divisible by 17 17 and 19 19 . Solving the appropriate Chinese Remainder Theorem equations, we see that n + a 0 ( m o d 17 ) , n + b 0 ( m o d 19 ) n 152 a 153 b ( m o d 17 × 19 = 323 ) n +a \equiv 0 \pmod{17} \;,\; n + b \equiv 0 \pmod{19} \hspace{0.5cm} \Leftrightarrow \hspace{0.5cm} n \; \equiv \; 152a - 153b \pmod{17\times19=323} and so it is only possible for two of these numbers to be divisible by 17 17 and 19 19 if n n is congruent to one of the numbers 12 13 14 15 16 17 31 32 33 34 50 51 95 112 113 114 129 130 131 132 133 146 147 148 149 150 151 152 164 165 166 167 168 169 170 183 184 185 186 187 202 203 204 221 265 266 282 283 284 285 299 300 301 302 303 304 \begin{array}{cccccccc} 12 & 13 & 14 & 15 & 16 & 17 & 31 & 32 \\ 33 & 34 & 50 & 51 & 95 & 112 & 113 & 114 \\ 129 & 130 & 131 & 132 & 133 & 146 & 147 & 148 \\ 149 & 150 & 151 & 152 & 164 & 165 & 166 & 167 \\ 168 & 169 & 170 & 183 & 184 & 185 & 186 & 187 \\ 202 & 203 & 204 & 221 & 265 & 266 & 282 & 283 \\ 284 & 285 & 299 & 300 & 301 & 302 & 303 & 304 \end{array} modulo 323 323 . It is easy to set up an Excel spreadsheet to test these options, and the smallest value of n n for which we get the desired divisibility for all eight numbers is n = 338 n = 338 (the second instance of n 15 ( m o d 323 ) n \equiv 15 \pmod{323} ). This makes the answer 2732 \boxed{2732} .

X X
May 15, 2018

The 8 numbers are from 338 to 345.

344 = 2 × 172 344=2\times172 339 = 3 × 113 339=3\times113 345 = 5 × 69 345=5\times69 343 = 7 × 49 343=7\times49 341 = 11 × 31 341=11\times31 338 = 13 × 26 338=13\times26 340 = 17 × 20 340=17\times20 342 = 19 × 18 342=19\times18

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