Re: resistors for leds
IF ALL LEDS WERE EQUAL AND HAVE EXACTLY THE SAME FORWARD DROP VOLTAGE it would be possible to use only one resistor to lit all leds. The calculation would be the following:
R = (5V - 3.6V)/(0.025A x 20 leds) = 1.4V / 0.5A = 2.8 ohm (considering the max value for the current in each led - anyway, don´t do that)
But returning to real world, the forward voltage of each led is not the same of other, so doing this way, probable some will be on and other will be off (if some of them are not going to burn, or going dead in sequence, due to current exceding the max value). In the best case, some will be brighter than the others.
To solve this, you should use a small value of resistance in series with each led, to compensate the variations in the characteristics of each led.