To find the current (I), we can use Ohm's Law:

P = I^2 * R

R can be simplified to the sum of R1 and R2 since they are in series:

R = R1 + R2
R = 20 ohms + 30 ohms
R = 50 ohms

Now we can rearrange Ohm's Law to solve for I:

I = sqrt(P / R)
I = sqrt(300 watts / 50 ohms)
I = sqrt(6)
I = 2.45 amperes

Therefore, the current flowing through the circuit is 2.45 amperes.