PhysicsPlease help
posted by Lynn .
R1= 10 ohm
R2= 20 ohm
R3= 50 ohm
R4= ???
R1, R2, R3 and R4 are connected in parallel across a 120 V "power" supply to generate a current of 21 Amps. The value of R4 is most nearly _____ ohms.
0, 5, 10, 16, 100, 200, 300
I think 5 ohms, but I'm not sure...In need of some assistance, please

R(combined) = 120/21 = 5.71 ohms
1/5.714 = 1/10 + 1/20 + 1/50 + 1/R4
= 0.175 + 1/R4
0.175 = 1/R4 + 0.170
1/R4 = 0.005
R4 = ___
It's not 5 ohms
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