Answer:
4: $16,000
5: $4978.66
6: .086278 or 8.62%
Explanation:
For the entirety of this problem I am going to assume that the interest is compounding annually
4: to find the interest earned in a period you have the following equation
(principle*(1+i)^n)-Principle
We can plug in the values we know
principle=p
(p*1.04)-p=640
This means that p=640/.04
p=16000
5: This is the same deal as the other one
2016=(p*(1.12)^3)-p
Solve this and get 1.404928p-p=2016
This means that .404928p=2016
Which means that p=4978.66
6: This one is slightly different but we can still use the same formula
720=4000((1+i)^2)-4000
We can add 4000 and then divide by 4000 to get
1.18=(1+i)^2
Take the square root of both sides and subtract 1 to solve for i
i= .086278 or 8.62%