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This problem can be solved in a very easy way, practically MENTALLY.
You only need to choose the major unknown variable by a rational way.
Let x be the money Sandra has (in dollars).
Then Nadine has 3x dollars and Harold has 2*(3x) = 6x dollars.
Together, they have x + 3x + 6x = 10x dollars, and this amount is exactly 357 dollars.
Hence, x = = 35.70 dollars. It is how much Sandra has.
Then Nadine has three times as much, i.e 3*35.70 = 107.10 dollars
and Harold has twice of it, i.e. 2*107.10 = 214.20.
CHECK. 35.70 + 107.10 + 214.20 = 357 dollars. ! Correct !
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Ideally, each problem you solve in school should teach you something.
Concretely, this given problem teaches you on how to choose the major unknown to make the solution easier.