# Thread: Solving basic equation by cross multiplying?

1. ## Solving basic equation by cross multiplying?

How do you go about solving this?

200/(r+10) = (200/r) + 1

2. ## Re: Solving basic equation by cross multiplying?

Originally Posted by benny92000
How do you go about solving this?
200/(r+10) = (200/r) + 1
That is the same as $\frac{200}{r+10}=\frac{r+200}{r}$

3. ## Re: Solving basic equation by cross multiplying?

Write:
$\frac{200}{r+10}=\frac{200}{r}+1$ as
$\frac{200}{r+10}-\frac{200}{r}-1=0$

Now, try to make a common denominator en know that a fraction becomes zero if the numerator becomes zero, you have also to assure that r is not -10 or r is not 0, because in that case you're dividing by zero what's not defined.

EDIT:
I tried to solve the equation and 40 is definetly not the right answer, there're no real solutions.
Is there a typo in it? And also, if you enter 40 int the equation you get:
$\frac{200}{50}=\frac{200}{40}+1\Leftrightarrow 4=6$ which is not true!
I think the equation has to be:
$\frac{200}{r+10}=\frac{200}{r}-1$

5. ## Re: Solving basic equation by cross multiplying?

Ok, but can you solve it now?

6. ## Re: Solving basic equation by cross multiplying?

Yes, I can, and I did. Thank you. I think I got -50 and 40.. So I rejected negative answer (unless he were driving his car backwards ).

7. ## Re: Solving basic equation by cross multiplying?

Yes, -50 and 40 are the solutions of the equation, but indeed it depends for which situation you're calculating this equation (if you have to reject a solution or not, you've to take in account a 'practical domain').