2. answer= 720
you can put the given information into the formula ir^t = f, where i is the initial amount, r is the rate of increase, t is time (in hours) and f is the amount after the given time. So you get 90(2)^h/4 = f (the rate being twice the amount, so 2; and this rate is for 4 hours, so divide the time by 4)
3. the first answer, the second term must be 10 to the power of something.
4. the first answer, it is the same reason as #3