Question

In a two-digit number, the units digit is twice the tens digit. If 9 is added to the number, the digits interchange their places. Find the number.

• A. 12
• B. 21
• C. 45
• D. 54

Answer 1

The correct option is A 12

Let the digit in the units place be $y$ and digit in the tens place be $x$.

$\Rightarrow$ The number is $10x+y$

Given: $y=2x---\left(i\right)$

and $10x+y+9=10y+x--\left(ii\right)$

Substitute the value of $y$ in $\left(ii\right)$

$\Rightarrow 10x+2x+9=10\left(2x\right)+x(\because y=2x)$

$\Rightarrow 12x+9=21x\Rightarrow 9x=9$

$\Rightarrow x=1$

Substitute the value of $x=1$ in $\left(i\right)$

$\Rightarrow y=2$

$\therefore x=1andy=2$

Therefore, number is 12.