Finding the Palindromes

What is the sum of the 2 palindromes that multiply to 436995? (Source: Mathcounts)

1221 990 1332 1440 1100

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2 solutions

Let the numbers be a b a \overline {aba} and c d c \overline {cdc} . Then

( 101 a + 10 b ) ( 101 c + 10 d ) = 436995 10201 a c + 1010 ( a d + b c ) + 100 b d = 436995 (101a+10b)(101c+10d)=436995\implies 10201ac+1010(ad+bc)+100bd=436995 .

Therefore either a a is 5 5 , c c is odd, or c c is 5 5 , a a is odd. Let us choose the first. Then, since 51005 c < 436995 , c 8 51005c<436995, c\leq 8 . Also, since 0 b , c , d 9 0\leq b, c, d\leq 9 , therefore c > 6 c>6 . So the only possible value of c c is 7 7 .

Therefore 357035 + 1010 ( 7 b + 5 d ) + 100 b d = 436995 101 ( 7 b + 5 d ) + 10 b d = 7996 357035+1010(7b+5d)+100bd=436995\implies 101(7b+5d)+10bd=7996 . Hence b b must be 8 8 , as the only possibility to get a unit's place digit 6 6 is from 7 , 7 × 8 = 56 7,7\times 8=56 . Then d = 4 d=4 .

Hence a b a = 585 , c d c = 747 \overline {aba}=585, \overline {cdc}=747 , and their sum is 585 + 747 = 1332 585+747=\boxed {1332} .

This problem has a few steps.

Step 1: Find the prime factorization of 436995.

3x3x3x3x5x13x83

Step 2: Use the numbers in the prime factorization to find the 2 palindromes that the problem is asking about.

3x3x83=747

3x3x5x13=585

Step 3: Add the palindromes you found in step 2.

747+585=1332

So the sum of the 2 palindromes that multiply to 436995 is 1332.

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