If you mean 10;100;1,000, then they are called <span>Powers of ten, and they're easy to represent as exponents.
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The answer to that is 9.91
Sometimes it will, and sometimes it won't.
I reason as follows:
(21 + x) + (30 + 2x) = 51 + 3x has an 'x' term
(42 + x) + (30 - x) = 72 has no 'x' term.
