In a Zener regulated power supply a Zener diode with V_z = 6.0 V is used for regulation. The load current is to be 4.0 mA and the unregulated input is 10.0 V. What should be the value of series resistor R_s?

This problem is an example problem in NCERT class XII text book.  I have copy pasted the solution below:

 

The value of R_S  should be such that the current through the Zener diode is much larger than the load current. This is to have good load regulation. Choose Zener current as five times the load current, i.e.,I_Z = 20 mA. The total current through R_S is, therefore, 24 mA. The voltage drop across R_S  is 10.0 – 6.0 = 4.0 V.  This gives R_S  = 4.0V/(24 × 10–3) A = 167 Ω. The nearest value of carbon resistor is 150 Ω. So, a series resistor of 150 Ω is appropriate.

Note that slight variation in the value of the resistor does not matter, what is important
is that the current I_Z should be sufficiently larger than I_L.