Physics-Please help

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

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  1. 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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