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Question:
Nine times a two-digit number is the same as twice the number obtained by interchanging the digits of the number. If one digit of the number exceeds the other number by 7, find the number.
Answer:

Let 10s digit of the number is x and unit digit of the number is y

So, the number = 10x + y

When the number is reversed = 10y + x

Given, nine times a two-digit number is the same as twice the number obtained by interchanging the digits of the number

=> 9(10x + y) = 2(10y + x)

=> 90x + 9y = 20y + 2x

=> 90x - 2x = 20y - 9y

=> 88x = 11y

=> y = 88x/11

=> y = 8x     .................1

Given, one digit of the number exceeds the other number by 7

=> y = x + 7

=> 8x = x + 7

=> 8x - x = 7

=> 7x = 7

=> x = 7/7

=> x = 1

Now, y = x + 7 = 1 + 7 = 8

So, the number = 10 * 1 + 8 = 10 + 8 = 18

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