Let's denote the amount invested at 5% as \(x\) and the amount invested at 4% as \(280 - x\).

The interest earned from the amount invested at 5% can be calculated using the formula: \( \text{Interest} = \text{Principal} \times \text{Rate} \times \text{Time} \).

Similarly, the interest earned from the amount invested at 4% can be calculated the same way.

Given that the total interest is N12.80 per annum and the interest from the 5% investment and the 4% investment should sum up to N12.80, we can write the equation:

\[5\% \text{ Interest} + 4\% \text{ Interest} = N12.80\]

\[0.05x + 0.04(280 - x) = 12.80\]

Now, let's solve for \(x\):

\[0.05x + 0.04(280 - x) = 12.80\]

\[0.05x + 11.20 - 0.04x = 12.80\]

\[0.01x = 1.60\]

\[x = \frac{1.60}{0.01} = 160\]

So, the amount invested at 5% is N160.00.