A man invested a sum of ₦280.00 partly at 5% and partly at 4%, if the total interest is ₦12.80 per annum, find the amount invested at 5%.

Let x be the amount invested at 5% and y be the amount invested at 4%. Then we have the following system of equations: x + y = 280 --- (1) (since the man invested a total of ₦280.00) 0.05x + 0.04y = 12.80 --- (2) (since the total interest earned is ₦12.80) To solve for x, we can use the first equation to get y = 280 - x, and substitute into the second equation to get: 0.05x + 0.04(280 - x) = 12.80 Simplifying and solving for x, we get: 0.05x + 11.20 - 0.04x = 12.80 0.01x = 1.60 x = 160 Therefore, the amount invested at 5% is ₦160.00.