Question

The second digit of a four digit number is "0". If you write the digits of this number backwards, you will get a number 9 times bigger than the original. What is the original number?

Answer 1

1089

Explanation:

The original number is A0CD, We have that DC0A = 9 x A0CD.

If A =1, then D = 9 and we have:

9C01 = 9 x 10C9

In order for this expression to be true, the following must also be true:

(C x 9) + 8 must be divisible by 10, which means that C x 9 must end in a 2.

The only multiple of 9 (from 9 to 81) that ends in a 2 is 72.

72 = 9 x 8.

Therefore, C must be 8 and the original number is 1089. Multiply it by 9 to check the answer:

1089 x 9 = 9,081

Therefore, 1089 is correct.