A student finds the average of ten 2 digits numbers. While copying numbers, by mistake, he writes in number with its digits interchanged. As a result his answer is 1.8 less than the correct answer. The difference of digits of the number, in which he made mistake is ?

Answer: Option A

Let us consider by mistake he writes 10th number with its digits interchanged.
$$\therefore \frac{10x+y-\left(10y+x\right)}{10}=18$$
(In this remaining nine numbers are same and they cancel out)
∴ 10x + y - 10y - x = 18
⇒ 9x - 9y = 18
⇒ x - y = 2