If you wire two 20-millihenry inductors in parallel, what is the inductance of the combination?

When inductors are wired in parallel, the total equivalent inductance decreases. This happens because the current has multiple paths to flow through, reducing the overall opposition to changes in current. The formula used for inductors in parallel is identical to the one used for resistors in parallel: 1/L_total = 1/L1 + 1/L2. A helpful shortcut is that when you have two identical inductors in parallel, the total inductance is simply half the value of one inductor. Thus, two 20-millihenry inductors in parallel result in an equivalent inductance of 10 millihenries. The 40-millihenry option is incorrect because that represents a series connection.