What is the sum of the 2 palindromes that multiply to 436995? (Source: Mathcounts)
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Let the numbers be a b a and c d c . Then
( 1 0 1 a + 1 0 b ) ( 1 0 1 c + 1 0 d ) = 4 3 6 9 9 5 ⟹ 1 0 2 0 1 a c + 1 0 1 0 ( a d + b c ) + 1 0 0 b d = 4 3 6 9 9 5 .
Therefore either a is 5 , c is odd, or c is 5 , a is odd. Let us choose the first. Then, since 5 1 0 0 5 c < 4 3 6 9 9 5 , c ≤ 8 . Also, since 0 ≤ b , c , d ≤ 9 , therefore c > 6 . So the only possible value of c is 7 .
Therefore 3 5 7 0 3 5 + 1 0 1 0 ( 7 b + 5 d ) + 1 0 0 b d = 4 3 6 9 9 5 ⟹ 1 0 1 ( 7 b + 5 d ) + 1 0 b d = 7 9 9 6 . Hence b must be 8 , as the only possibility to get a unit's place digit 6 is from 7 , 7 × 8 = 5 6 . Then d = 4 .
Hence a b a = 5 8 5 , c d c = 7 4 7 , and their sum is 5 8 5 + 7 4 7 = 1 3 3 2 .