Modular guessing?

A B C D 11 ( m o d 15 ) \overline{ABCD} \equiv 11 \pmod{15}

A 1 ( m o d 7 ) A \equiv 1 \pmod{7}

A B 0 ( m o d 7 ) \overline{AB} \equiv 0 \pmod{7}

C + A 0 ( m o d 7 ) C + A \equiv 0 \pmod{7}

C D 1 ( m o d 5 ) C \equiv D \equiv 1 \pmod{5}


A B C D = ? \overline{ABCD} = ~?


The answer is 1466.

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1 solution

Zyberg Nee
Jun 27, 2016

Since we have A B C D 11 ( m o d 15 ) \overline{ABCD} \equiv 11 \pmod{15} , we know that 15 divides A B C D 11 \overline{ABCD} - 11 evenly.

From A 1 ( m o d 7 ) A \equiv 1 \pmod{7} we know that A = 1 / 8 A = 1 / 8 .

From A B 0 ( m o d 7 ) \overline{AB} \equiv 0 \pmod{7} we know that i f A = 1 , t h e n B = 4 ; i f A = 8 , t h e n B = 4 if~A = 1,~then~B = 4;~if ~A = 8,~then~B =4 . We conclude that B = 4 B = 4

From C D 1 ( m o d 5 ) C \equiv D \equiv 1 \pmod{5} we know that C = 1 / 6 C = 1 / 6 , D = 1 / 6 D = 1 / 6

From C + A 0 ( m o d 7 ) C + A \equiv 0 \pmod{7} we can work out C C . All possible values of C + A C + A are: 1 + 1 = 2 ; 1 + 8 = 9 ; 6 + 1 = 7 ; 6 + 8 = 14 1 + 1 = 2;~1 + 8 = 9;~6 + 1 = 7;~ 6 + 8=14 . We conclude that C = 6 C = 6 .

Now we know that our number looks like this: A 46 D \overline{A46D} . There are four numbers that can be made up with currently possible values of A A and D D , however, only one of them is evenly divisible by 15 15 when it is subtracted by 15.

Possible numbers are: 1461 ; 1466 ; 8461 ; 8466 1461; 1466; 8461; 8466 .

Answer: 1466 \boxed{1466}

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