Answer:
r = 1 Ohms
Step-by-step explanation:
Given that:
P =

where r is the resistance in the circuit.
If r = 1 Ohms;
P =

=

=

= 15.1235
P = 15.12 W
If r = 3 Ohms;
P =

=

P = 10.18 W
If r = 5 Ohms;
P =

=

= 7.230
P = 7.23 W
Therefore, the value of r for maximum power is 1 Ohms.