Find the Number!

Level 2

Find the 3-digit number which satisfies the following conditions.

  • The number is a multiple of 4.

  • The reverse of the 3-digit number is 495 greater than the number itself.

  • If you add 36 to the number, the tens and ones digit will swap with each other.


The answer is 348.

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1 solution

David Vreken
Apr 23, 2019

Let the 3 3 -digit number be X Y Z = 100 X + 10 Y + Z \overline{XYZ} = 100X + 10Y + Z for single digit numbers X X , Y Y , and Z Z .

Since adding 36 36 to the number make the tens and ones digit will swap with each other, 10 Y + Z + 36 = 10 Z + Y 10Y + Z + 36 = 10Z + Y , which solves to Z = Y + 4 Z = Y + 4 . Since Y Y and Z Z are single digits, the possibilities for ( Y , Z ) (Y, Z) are ( 0 , 4 ) (0, 4) , ( 1 , 5 ) (1, 5) , ( 2 , 6 ) (2, 6) , ( 3 , 7 ) (3, 7) , ( 4 , 8 ) (4, 8) , and ( 5 , 9 ) (5, 9) .

Since 100 X + 10 Y + Z 100X + 10Y + Z is a multiple of 4 4 , and 100 X 100X is a multiple of 4 4 , 10 Y + Z 10Y + Z must also be a multiple of 4 4 . The possibilites for ( Y , Z ) (Y, Z) are now further limited to ( 0 , 4 ) (0, 4) and ( 4 , 8 ) (4, 8) .

Since the reverse of the 3 3 -digit number is 495 495 greater than the number itself, 100 X + 10 Y + Z + 495 = 100 Z + 10 Y + X 100X + 10Y + Z + 495 = 100Z + 10Y + X , which solves to Z = X + 5 Z = X + 5 , so Z 5 Z \geq 5 . The possibilites for ( Y , Z ) (Y, Z) is now limited to ( 4 , 8 ) (4, 8) , which makes X = 3 X = 3 , Y = 4 Y = 4 , and Z = 8 Z = 8 for the number 348 \boxed{348} .

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