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Homework Statement:

A zener regulator has an input voltage in the range 15V20V and a load current in the range of 5mA20mA. If the zener voltage is 6.8V, the value of the series resistor should be:
(a)390Ω
(b)420Ω
(c)440Ω
(d)460Ω
Relevant Equations:
 Stuff
This is a very confusing situation for me. If the input voltage were constant, then I would be able to understand that by varying load resistance, I can change the current through it, such that the drop across the load = Vz = 6.8V.
But the input voltage is also not constant. So what am I to understand? The 520mA range is for what voltage?
I only know:
If Vs is source voltage, then
Vs I*Rs  6.8=0
So, I*Rs = Vs6.8
So, Imax = (206.8)/Rs
Imin = (156.8)/Rs
This is total current through series resistor. Some of it will run through zener and some through load. But load itself is variable.
Please show me some way.
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