LED¶

Vforward = 2V
Iforward = 20mA = 0,020A

R=2/0,02=100Ohm
U2=I*R2=100*0,02=2V

I= U/(R1+R2)
R1+R2=U/I
R1 = (U / I ) - R2 = (3/0,02)-100 = 50

R=50+100=150
I=3/150=20mA

Just double it:
R=100+100
I = 3/200 = 0,015

Just tribble it:
I = 3/250 = 0,012

Just 4 times:
I = 3/300 = 0,010

R1= ( Ug / If ) - ( Uf / If ) = (Ug-Uf)/If

U
---
R*I

R1 R2
-- = --
U1 U2