A man invested a sum of N280.00 partly at 5% and partly at 4%. if…

A man invested a sum of N280.00 partly at 5% and partly at 4%. if the total interest is N12.80 per annum, find the amount invested at 5%

Correct Answer: Option D

Explanation

Let the amounts invested at 4% and 5% respectively be x and y.

(therefore x + y = 280 … (i))

Interest on x = (frac{x times 4 times 1}{100} = 0.04x)

Interest on y = (frac{y times 5 times 1}{100} = 0.05y)

(therefore 0.04x + 0.05y = 12.80)

(implies 4x + 5y = 1280 … (ii))

From (i), (x = 280 – y).

Put into (ii), (4(280 – y) + 5y = 1280)

(1120 – 4y + 5y = 1280)

(1120 + y = 1280 implies y = 1280 – 1120 = N160)

(therefore) N160 was invested at the rate of 5% per annum.