Daughters homework challenge.

[chrispick77]

Using the digits 1-8 how many different ways can the digits be arranged to make 2 numbers whose sum is 9999.

I've had a go at my 9 yr old daughters homework challenge and think the answer is 24.
It's been 35 years since I was 9 so forgive me if it's wrong but I want to explain to her how I got 24 IF it's correct.
Could someone help us out please?

[Guest]

I would start by think of numbers that add to that "9" in each column- 1+8, 2+7, 3+ 6; 4+ 5; 5+ 4; 6+ 3; 7+ 2; 8+ 1. Notice that two such integers cannot add to "19" so there is no "borrowing".