With a load resistance of 10 ohms, the efficiency of the current source is 70%. What is the internal resistance of the source?

given
load resistance R = 10 Ohm
Efficiency = 70% = 0.7
find the internal resistance r
decision
(Efficiency) = R / (R + r) from here we find r
efficiency * (r + R) = R
efficiency * r + efficiency * R = R
r * efficiency = R – R * efficiency
r * efficiency = R * (1 – efficiency)
r = (R * (1 – efficiency)) / efficiency
substituting the known data, we get
r = (10 Ohm * (1 – 0.7) / 0.7) = (10 Ohm * 0.3) / 0.7 = 3 Ohm / 0.7 = 30/7 Ohm = 4.3 Ohm
answer: 4, 3 ohm