Find the current through the $40\text{ }\Omega$ resistor in the given circuit containing an ideal diode, three resistors, and two cells.
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Once you determine that the diode is reverse-biased, you can mentally remove the top branch entirely. The circuit becomes a simple loop with an effective voltage of $15\text{ V}$ across a total resistance of $70\text{ }\Omega$, giving a current of $\frac{15}{70} \approx 0.21\text{ A} $ in a few short steps.
