Since this is a compound interest problem, you have to take note that the amount Catherine will get per year is not the same. It will increase per year since it is compounded. So first, we get the amount after one year. This will be 7000 x 0.04 which is 280 plus 7280. In the second year, she will get 7571 (7280 x 0.04 + 7280). In the third year, she will get 7874 (7571 x 0.04 + 7571). In the fourth year, she will get 8189 (7874 x 0.04 + 7874). And finally in the fifth year, she will get 8517 (8189 x 0.04 +8189). So after five years, she has 8517
5/6 is always going to my equal to itself. o divide the two and see what you get
Check the picture below.
now, keep in mind that ship B is going at 20kph, thus from noon to 4pm, is 4 hours, so it has travelled by then 20 * 4 or 80 kilometers, thus b = 80.
whilst the ship B is moving north, the distance "a" is not really changing, and thus is a constant, that matters because the derivative of a constant is 0.