In the circuit shown (above, right) current through $$2R$$ is $$3\text{ A}$$. Find the current (in A) through $$R$$ if a $$24\text{ V}$$ cell replaces the $$60\text{ V}$$ cell.
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The three resistors $$R$$ are in series ($$3R$$), and this branch is in parallel with $$2R$$ across the cell. With $$60\text{ V}$$, the current in $$2R$$ is $$\frac{60}{2R} = 3$$, so $$R = 10\,\Omega$$. With $$24\text{ V}$$, the current through the $$R$$ branch is $$\frac{24}{3R} = \frac{24}{30} = 0.8\text{ A}$$.