Find the number of positive four-digit integers between 1000 and 9999 (inclusive) such that is increased by 2088 when the digits of are reversed.
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Let x = 1 0 0 0 a + 1 0 0 b + 1 0 c + d , where a , b , c , d = 0 , 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 AND a = 0 . The problem is interested in the condition:
1 0 0 0 a + 1 0 0 b + 1 0 c + d + 2 0 8 8 = 1 0 0 0 d + 1 0 0 c + 1 0 b + a ;
or − 9 9 9 a − 9 0 b + 9 0 c + 9 9 9 d = 2 0 8 8 ;
or 1 1 1 ( d − a ) + 1 0 ( c − b ) = 2 3 2 .
The Diophantine equation 1 1 1 α + 1 0 β = 2 3 2 only has one solution in positive integers: ( α , β ) = ( 2 , 1 ) . Hence, we require d − a = 2 and c − b = 1 which generates the ordered pairs:
( 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 ) , or 9 pairs.
which gives a grand total of 7 ⋅ 9 = 6 3 4-digit numbers that satisfy this condition.