= Solution
Approximate each source as localized, keep the angular factors unchanged and use inverse-square flux dilution. The increased distance gives
$$
R_{120}\simeq R_{80}\left(\frac{80}{120}\right)^2=1.54\frac49=\boxed{0.684\,\mathrm{nm\,s^{-1}}}.
$$
This carries the composition/thickness convention of the previous answer. A greater source distance reduces fractional distance and angle variation across a given substrate, improving composition uniformity at the cost of rate. A real <magnetron sputtering> target is extended, with an angular emission distribution, and gas scattering can change the distance law; this is an estimate rather than a calibrated prediction.
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