Question 247059
Find R1 when R is 210. 

R =     84
    ---------
   R1/2 + R1/3
   
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Cross-multiply:
210(R1/2 + R1/3) = 84
105R1 + 70R1 = 84
175R1 = 84
R1 = 0.48
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Cheers,
Stan H.