Question 69687
1/R = 1/R1 + 1/R2 + 1/R3 where R = 20, R2 = 40 and R3 = 200 
1/20 = 1/R1 + 1/40 + 1/200
Then:
1/R1 = 1/20-1/40-1/200
1/R1 =  [10 - 5 -1]/200
1/R1 = 4/200
1/R1 = 1/50
R1 = 50
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Cheers,
Stan H.