# Math Help - pre-alg rational equations

1. ## pre-alg rational equations

2/7m = 8/21 - I'm just wading into Pre-algebra and need an explanation to solving this.

2. Originally Posted by SonyaD
2/7m = 8/21 - I'm just wading into Pre-algebra and need an explanation to solving this.
Is this (2/7)m= 8/21 or 2/(7m)= 8/21? Please use parenthese to make your meaning clear.

You solve an equation for the unknown value by "undoing" what has been done to it. Assuming the problem is (2/7)m= 8/21, two things have been done to m- it has been multiplied by 2 and divided by 7. We "undo" multiplication by 2 by doing the opposite: divide both sides by 2: (2/7) divided by 2 is 1/7 ane 8/21 divided by 2 is 4/21: (1/7)m= 4/21. We "undo" division by 7 by doing the opposite: multiply both sides by 7: (1/7)(7)= 1 and (4/21)(7)= 4/3: x= 4/3. Now that we have the "x" by itself on one side of the equation, we can just read off the answer: x is equal to 4/3.

If this were 2/(7m)= 8/21 you could start out the same: dividing both sides by 2 gives 1/(7m)= 4/21 and multiplying both sides by 7 gives 1/m= 4/3. Now what remains is that m has been "inverted". The opposite of "inversion" is just inversion again- turn 0ver both fractions to get x/1= x= 3/4.