/--o   U2=30V
 |
 R1
 |
 +--o   U1=5V
 |
 R2
 |
 \--o   GND=0V



R1   U2-U1
-- = -----
R2    U1


R1   30V-5V
-- = ------ = 5
R2     5V


R1 = 5 * R2