1.  If the digit in the unit's place of a two-digit number is halved and the digit in the ten's place is doubled, the number thus obtained is equal to the number obtained by interchanging the digits. Which of the following is definitely true ?
A. Sum of the digits is a two-digit number.
B. Digit in the unit's place is twice the digit in the ten's place.
C. Digits in the unit's place and the ten's place are equal.
D. Digit in the unit's place is half of the digit in the ten's place.
E. None of these.

Answer: Option B

Explanation:

Let the ten's digit be X and unit's digit be Y.

Then, 10 x 2X + Y/2 = 10Y + X

⇒ 20X - X = 10Y - Y/2

⇒ 19X = 19Y/2

⇒ Y = 2X

Thus, the unit's digit is twice the ten's digit.