求基本電學等效電阻?

Let's start from A to B.
In the 1st loop, 40-ohm and 60-ohm resistors in paralle. Equivalent resistance
= 40 x 60/(40+60) ohms = 24 ohms

In the 2nd loop, 20-ohm and 50-ohm resistors in series.equivalent resistance
= (20 + 50) ohms = 70 ohms
This 70-ohm resistance is in parallel with the 30-ohm resistor. Equivalent resistance
= 70 x 30/(70 + 30) ohms = 21 ohms.

Because the two loops are in series, hence equivalent resistance of the two loops
= (24 + 21) ohms
= 45 ohms