The resistance of the unknown resistor is determined as 0.39 ohms.
The total resistance of the circuit is calculated as follows;
V = IR
R = V/I
R = (3.5)/10
R = 0.35 ohms
The resistors are in parallel arrangement
1/R = 1/R₁ + 1/R₂ + 1/R₃
1/0.35 = 1/5 + 1/10 + 1/R₃
100/35 = 3/10 + 1/R₃
20/7 = 3/10 + 1/R₃
1/R₃ = 20/7 - 3/10
1/R₃ = (200 - 21)/70
1/R₃ = 179/70
R₃ = 70/179
R₃ = 0.39 ohms
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