# Math help with investments

#### polishp86

##### The Man, DDS
10+ Year Member
5+ Year Member
Ok guys here is my question, from the topscore pro software, test one, math question 19.

Mark has \$7200 invested, part at 4% and the rest at 5%. if the annual income from both investments is the same, what is the income from his investment?

Let x = principle amt invested at 4%
let 7200 - x = principle amt invested at 5%
principle x rate = income

.04x = .05 (7200-x)

9x = 5(7200)

x=4000

4000 x .04 = income

income = 160

total = 160 x 2 = 320

I get all of the above, EXCEPT for where the 9x = 5(7200) is coming from. I know it's probably some simple calculation, but I am not seeing it for the life of me, and my test is in 2 days! crap!!!!!! I also do not understand how the -x has magically disappeared after the 7200.

anyways, I would appreciate any explanation for this, the topscore software doesn't do that great of a job explaning how it gets to certain conclusions/answers.

#### DrYoda

##### Space Cowboy
10+ Year Member
It looks like they multiplied by 100 to get the decimals as whole numbers, distributed the 5 (previously 0.05) and then moved the 5x to give 9x.

0.04x=0.05(7200-x) multiply by 100 to get whole numbers

4x=5(7200-x) distribute

4x= 5*7200-5x add 5x to each side

9x=7200*5 divide each side by 9

x=4000