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Find the number of positive four-digit integers x x between 1000 and 9999 (inclusive) such that x x is increased by 2088 when the digits of x x are reversed.

48 63 56 72

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1 solution

Tom Engelsman
Feb 16, 2019

Let x = 1000 a + 100 b + 10 c + d x = 1000a + 100b + 10c + d , where a , b , c , d = 0 , 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 a, b, c, d = {0,1,2,3,4,5,6,7,8,9} AND a 0. a \neq 0. The problem is interested in the condition:

1000 a + 100 b + 10 c + d + 2088 = 1000 d + 100 c + 10 b + a 1000a + 100b + 10c + d + 2088 = 1000d + 100c + 10b + a ;

or 999 a 90 b + 90 c + 999 d = 2088 ; -999a - 90b + 90c + 999d = 2088;

or 111 ( d a ) + 10 ( c b ) = 232. 111(d-a) + 10(c-b) = 232.

The Diophantine equation 111 α + 10 β = 232 111\alpha + 10\beta = 232 only has one solution in positive integers: ( α , β ) = ( 2 , 1 ) . (\alpha, \beta) = (2,1). Hence, we require d a = 2 d - a = 2 and c b = 1 c - b = 1 which generates the ordered pairs:

( a , d ) = ( 1 , 3 ) ; ( 2 , 4 ) ; ( 3 , 5 ) ; ( 4 , 6 ) ; ( 5 , 7 ) ; ( 6 , 8 ) ; ( 7 , 9 ) . (a,d) = (1,3); (2,4); (3,5); (4,6); (5,7); (6,8); (7,9). , or 7 pairs

( b , c ) = ( 0 , 1 ) ; ( 1 , 2 ) ; ( 2 , 3 ) ; ( 3 , 4 ) ; ( 4 , 5 ) ; ( 5 , 6 ) ; ( 6 , 7 ) ; ( 7 , 8 ) ; ( 8 , 9 ) (b,c) = (0,1); (1,2); (2,3); (3,4); (4,5); (5,6); (6,7); (7,8); (8,9) , or 9 pairs.

which gives a grand total of 7 9 = 63 7 \cdot 9 = \boxed{63} 4-digit numbers that satisfy this condition.

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