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Textbook asked student to solve the following simultaneous equations where x is distance in cm and t is time in minutes:

  1. x = 2t
  2. x = -4t -20

I multiplied equation 1 by the 't' coefficient of equation 2, -4, and vice-versa multiplied equation 2 by the 't' coefficient of 1, 2, to get:

  1. -4x = -8t
  2. 2x = -8t-40

I then took the second equation from the first to get:

-6x = -40 so solved x to be -40/-6 or 6.67.

Substituting this value of x into the original equation 1 gave t to be 3.33.

However, the textbook answer was x= -6.67 and t= -3.33.

Where did I go wrong?

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    I don't understand why you did the math that way...? x = x, therefore 2t = -4t - 20. Multiplying both sides by different factors to eliminate t just makes this harder. – JMac Feb 27 '19 at 13:30
  • @JMac That is the difference between blindly following a procedure to eliminate $t$ that you have learned, and understanding what you are doing ;) (Unfortunately, even following the procedure doesn't work if you make a mistake, as the OP found out!) – alephzero Feb 27 '19 at 13:43

1 Answers1

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When you "take the second equation from the first", you should end up with $6x=-40$, not $-6x=-40$.

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