JAMB 1992 · UME · Q13

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 $5\%$ .

Worked solution (try it first)

Let $x$ be the amount at $5\%$ .
Then $280 - x$ is at $4\%$ .
The interest adds up to 12.80: $0.05x + 0.04(280 - x) = 12.8$ , which is $0.01x + 11.2 = 12.8$ .
So $0.01x = 1.6$ and $x = 160$ .
He invested ₦160.00 at $5\%$ , option D.