$325.06+$198.78 how can you check this by casting out the nines?

Answer 1

3+2+5+6 = 16, which is congruent to 7 modulo 9. Hence 32,506 is also congruent to 7 modulo 9.

1+9+8+7+8 is congruent to 6 modulo 9. Hence so is 19,878.

So their sum is congruent to 13 modulo 9, which is congruent to 4.

The actual sum of those numbers, as I do it in my head, seems to be $523.84 5+2+3+8+4=22, which is congruent to 4 modulo 9.

Good. I seem to have done it right. (This assumes that any error I was likely to make would have just gotten a digit off 1 or 2, and hence would lead to failing the casting-by-nines check.)