The reading of ammeter in the following circuit. (Assuming forward voltage drop across diode is zero)

1^st Zener diode is forward biased hence it will behave as normal diode.

2^nd Zener diode is reverse biased. 

Voltage drop across resistor , $V=20-15=5V$

Therefore, Current in circuit, $I=\frac{V}{R}=\frac{5}{10000}=0.5\times {10}^{-3}A=0.5mA$