The step that involves the addition of the units digits and multiplying the result with one more than the remaining digits is used in which method?

Q. In the addition of two $3$-digit numbers above, R and T represent two different digits and the units digit of the answer is zero. What digit does T represent?

Q. A number is $27$ more than the number obtained by reversing its digits. If its units and tens digits are $x$ and $y$ respectively, write the linear equation representing the above statement.

Q. The units digit of a two-digit number is $2$ more than the tens digit. If the number is subtracted from the sum of the squares of its digits the result is two-thirds of the product of the digits. What is the number?

Q. How many numbers with not more than three digits can be formed using only the digits $1$ through $7$ with no digit used more than once in a given number?

Q. The total number of numbers of not more than $20$ digits that are formed by using the digits $0,1,2,3$ and $4$. is