Doris invested some money at 7% and some money at 8%. She invested $4,000 more at 8% than she did at 7%. Her total yearly interest from the two investments was $920. How much did Doris invest at each rate?
A)$100 at 7%, $7,900 at 8%
B)$5,000 at 7%, $8,000 at 8%
C)$4,000 at 7%, $9,000 at 8%
D)$4,000 at 7%, $8,000 at 8%
E)$4,500 at 7%, $7,500 at 8%

Question

Quizplus · Accepted Answer

Let x be the amount invested at 7%. Then the amount invested at 8% is x + $4,000.

The interest from the 7% investment is 0.07x, and the interest from the 8% investment is 0.08(x + $4,000), since that investment is $4,000 more.

The total interest is $920. So we can write an equation:

0.07x + 0.08(x + $4,000) = $920

Simplifying:

0.07x + 0.08x + $320 = $920

0.15x = $600

x = $4,000

So Doris invested $4,000 at 7% and $8,000 ($4,000 + $4,000) at 8%. The answer is D.

Doris Invested Some Money at 7% and Some Money at 8