Please take a look at my work?

[DrPepper]

Hello! I am currently working on some problems...On problems 15 (% solution problem), 16 (interest problem), 18, and 19, I have reached an impasse. I have the answers to all of them, but need help figuring out how on all of them...I have scanned my work and attached it, if someone could please take a look.Thank you very very very much! Any help at all is appreciated Here are the instructions for #16:

 

[DrPepper]

After taking another look at problem 15 again, I have gotten the correct answer: 4.5L

I also attempted number 16 again, and have gotten the answer of 22,000, which I believe is the correct answer.

For problem number 18, I have reached the correct answer as well, 35-207i
by 49-63i-63i+81i^2+63-81i+4
which equals 35-207i

for 19, I am still somewhat confused...
x^3+64=0
subtract 64 from both sides
x^3 = -64
then the cube root of -64 is negative 4
I cannot seem to figure out how to get the second answer

 

[wjm11]

x^3+64=0
subtract 64 from both sides
x^3 = -64
then the cube root of -64 is negative 4
I cannot seem to figure out how to get the second answer

Since -4 is one of the roots of the equation, that means that (x + 4) is a factor of (x^63 + 64). Divide (x^63 + 64) by (x + 4) to obtain another factor, which will be a quadratic. Then use the quadratic formula.