1601
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It is not possible to have any integral multiples of 1600 whose sum is -1. The sum can be 0 or any multiple of 1600, but other sums are not possible.
If non-integral multiples are permitted then there are an infinite number of possible answers.
multiples of 400: 400, 800, 1200, 1600, 2000, 2400, ...
That the sum of its digits add up to 9 as for example 9*9 = 81 and 8+1 = 9
Oh, isn't that a happy little math problem! We take the sum of all the odd multiples of five from 1 to 2007, which is 5 + 15 + 25 + ... + 2005. This sum is 100,040,000. When we divide this by ten, we get a remainder of 0, like a peaceful little stream flowing gently through the forest.
If half of the sum is 90 then the full sum must be 180. The three consecutive multiples of 10 which add up to 180 are 50, 60 and 70.
There are infinitely many multiples of 9 and it is not possible to add them all.