How much money was invested in the CD​ account?

Question

A mother earned $15000.00 from royalties on her cookbook. She set aside 20% of this for a down payment on a new home. The balance will be used for her son's future education. She invests a portion of the money in a bank certificate of deposit (CD account) that earns 4% and the remainder in a savings bond that earns 7%. If the total interest earned after one year is $720.00 , how much money was invested at each rate?

Answer 1

80% of 15,000 = .08(15000)= 12000

divided between CD's at 4% and savings bond at 7% which together earn $720 in interest the first year.

.07x + (12000-x).04 = 720

.07x -.04x = 720 - .04(12000)

.03x = 720 -480 = 240

x = 240/.03= $8000 invested in CDs

12000-8000= $4,000 invested in savings bonds

average rate of return on both is 720/12000= 72/1200 = 18/300 = 6/100= 6%

7-6 = 1

6-4 = 2

2/1=2 times as much will be invested in CDs

x/(12000-x) = 2

24000 -2x = x

3x = 24000

x = $8000

12000-x =$4000

Answer 2

Let x be the amount put into a CD at 4% interest

Let y be the amount put into a savings bond at 7% interest.

After setting aside 20% of $15000, the mother has $12000 left to invest.

0.04x + 0.07y = 720

x+y = 12000 --> y = 12000-x

0.04x + 0.07(12000-x) = 720

0.04x + 840 - 0.07x = 720

Can you solve for x and then y?