Find R2  when Req = 800 and R1=1200. Req = R1 * R2 /R1 + R2 Is there a solution to this problem?    

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If you substitute the values into the equation, you get:800 = 1200 * R2/1200 + R2The first step I would do is to multiply both sides of the equation by (1200 + R2). This will get rid of the fraction.(1200 + R2)*800 = (1200*R2/1200 + R2) * (1200 + R2)On the left side of the equation you multiply 800 by both the terms inside the parentheses to get 960,000 + 800 R2. On the right side, the 1200 + R2 on the bottom of the fraction cancels with the 1200 + R2 that you are multiplying to leave just 1200*R2. So now the equation looks like this:960,000 + 800R2 = 1200R2Next, subtract 800R2 from both sides of the equation to isolate your variable on one side. Subtracting 800R2 from the left side of the equation leaves just 960,000. Subtracting 800R2 from the right side leaves 400R2 -- (1200R2 minus 800R2 = 400R2). So now the equation looks like this:960,000 = 400R2Finally, to get R2 alone, divide both sides of the equation by 400. You get R2 = 2400.