A number is a two-digit number such that the sum of its digits is 7.

How many two-digit numbers are there such that the sum of their digits is 7?

In this problem:

A number is a two-digit number such that the sum of its digits is 7.

There are only two possibilities: either the first digit is odd and the second digit is even, or vice versa. In other words, there are two cases to consider:

Case 1: The first digit is odd and the second digit is even. In this case, we have “n” = (2x + 1) + (2y – 1). Since 2x + 1 + 2y – 1 = 7, we have y = 4 – x. The value of n in this case is therefore 22x + 2(4 – x). The number of such numbers is 2(22x + 2(4 – x)) = 2(22x + 8).

Last modified: August 18, 2022