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word problems and turning them into algebraic equation to solve

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abc is a any 3-digit number
abc can be written as (100a+10b+c) because a is in the hundreds place, b is in the tens place, and c is in the ones place
for example  684=(600)+(80)+4)
proof: abc=(100a+10b+c)
                =(99a+a)+(9b+b)+(c)
                =(99a+9b)+(a+b+c)
if two numbers are both divisible by 9 then their sum is divisible by 9
(99a+9b) is divisible by 9 because it can be written as 9(11a+b)
therefore if (a+b+c) is divisible by 9 then (99a+9b)+(a+b+c) is divisible by 9 and we know
that (99a+9b)+(a+b+c)=100a+10b+c=abc, a 3-digit number