Answer: a = 10
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Work Shown:
Therefore, a = 10
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As an alternative approach, we could work in reverse like this:
Then notice that the 20+a must be 30, so solving 20+a = 30 leads to a = 10
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As yet another alternative approach (well I suppose it's actually 2 extra approaches) we could use either polynomial long division or synthetic division. Either way, you should end up with a remainder of 10, which corresponds directly to a = 10