Step 1: State how radiated power depends on length and wavelength.
For a linear antenna the effective radiated power scales as the square of the length-to-wavelength ratio, \[ P \propto \left(\frac{l}{\lambda}\right)^2 \]
Step 2: Write the ratio of new to old power.
Comparing the new state (2) with the old state (1), \[ \frac{P_2}{P_1} = \left(\frac{l_2}{l_1}\cdot\frac{\lambda_1}{\lambda_2}\right)^2 \]
Step 3: Translate the percentage changes.
A 60 percent longer antenna means $l_2 = 1.6\,l_1$, and a 20 percent shorter wavelength means $\lambda_2 = 0.8\,\lambda_1$.
Step 4: Substitute these factors.
\[ \frac{P_2}{P_1} = \left(\frac{1.6}{0.8}\right)^2 = (2)^2 = 4 \] so the new power is four times the old.
Step 5: Convert a ratio of 4 into a percentage increase.
The increase is the new value minus the old, divided by the old, times one hundred, \[ \%\,\text{increase} = (4 - 1)\times 100 \]
Step 6: Evaluate.
This gives a 300 percent increase, which is option 3.
\[ \boxed{300\%} \]