There is an infinite amount of numbers that can sum up to 24. Only 23, if you are just counting positive numbers.
-21
To make this simpler, I'll assume that numbers with a leading zero like 0179 are "four digit numbers" for the purposes of this question. The number of such numbers that can be formed by those numbers without repeating one of the digits is 4x3x2x1 = 24. We could list them all and add them up, but let's instead just realize that we can choose any digit and put it in any position, and the other digits can then be arranged in 3x2x1 = 6 ways. So the sum for any position is (0+1+7+9)*6 = 102, and the sum of the set of "four digit" numbers is 102x1000+102x100+102x10+102 = 113322. You can figure out for yourself what the sum would be if you exclude the numbers that are really three digit numbers (hint: use the technique above to find the sum of the three digit numbers that can be formed using the digits 1, 7, and 9; then subtract that from the other total).
18 & 42
12
There are three such numbers: 12, 24 and 36.
24
24 if you don't repeat any numbers.24
24 of them.
24
Two numbers have a product of 80 their sum is 24?
-- If you accept 4-digit numbers with 'zero' as the first digit, thenthere are 24 possibilities, and their sum is 119,988 .-- If the first digit can't be a 'zero', then there are 18 possibilities,and their sum is 115,992 .Note: I didn't repeat any digits.-- If you allow repeated digits, and accept leading zeros, then there are 256 possibilities,and their sum is 1,279,872.-- If you allow repeated digits but not leading zeros, then there are only 192 possibilities,and their sum is 1,247,904.
27
24
24 different numbers.
24 = 4*3*2*1
24 = 16