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Cross Multiplication
Cross Multiplication for ProportionalitiesAn electrician needs to be able to use proportionality to solve a problem about conductor resistance at length. Here's how it works. If a certain conductor has a resistance of 18 ohms per 1000 feet, what will the resistance be for, say, 500 feet? 9 ohms, you come up with. Right. But if the length is not an exact half, or some other common fraction, how is the solution determined?Cross multiplication can do it.A over B equals C over DOr, A times D divided by B equals C. Sound difficult? Let me explain.If a conductor is (A)18 ohms per (B) 1000 feet, then what is the resistance (C) at (d)500 feet?So it is A times D divided by B equals C.Or 18 times 500 divided by 1000 equals 9.Works every time.Find practice problems HERE.Find audio support HERE.
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