Answer:
.513 Ω
Step-by-step explanation:
The current through the 20 Ω load can be found to be
(12-.3) v / 20 Ω = .585 A
This current also flows through the internal resistance and produces the .3 v drop across this resistor
V/I = R
.3 v / .585 A = .513 Ω internal resistance