Let us use Thevenin's Theorem. Assume there is Load Resistance in the arms where we need to calculate the current I. Solve for Thevenin's voltage and Resistance. After everything. We draw equivalent Thevenin's circuit. In this circuit, we can assume load resistance as zero and get the current by simply dividing Thevenin's Voltage by Resistance. We easily see that reciprocal theorem for electric circuits is satisfied. The reciprocity means that changing the position of battery from one place two other reverses the current value in the original position. This condition is satisfied here.