Remember that when the ZD is in RB it will keep everything that's in parallel with it at the same voltage as itself (approx. 12V, in the present case).
The voltage on the right side of R1 is 12 V, which is enough to close the ZD.
Once it's closed, the block on the right side will stay with 12 V, too.
The total current is 12/470 = 25.5 mA
The current on R2 is 12/490 = 24.5 mA
Because the ZD is IN PARALLEL with R2 (490), the current on the ZD is 25.5-24.5=1 mA.
