Find R1 when R is 210.  (84 and R1/2 +R1/3 is in faction form) R = 84   R1/2 + R1/3 210 = 84 R1/2 + R1/3 I don't know how to work this with problem. Please help.    

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So.. if I understand what you're trying to say correctly, you have the equation R = 84/(R1/2 + R1/3), and you want to solve for R1 when R = 210?  So just try to get R1 alone:210 = 84/(R1/2 + R1/3)    (multiply both sides by (R1/2 + R1/3) to get it out of the denom.)210*(R1/2 + R1/3) = 84   (divide by 210)R1/2 + R1/3 = 84/210  (multiply by 2*3 = 6 to get rid of fractions)3R1 + 2R1 = 84*6/210  (reduce and combine terms)5R1 = 12/5  (divide by 5 to get R1 alone)R1 = 12/25