y = (1+r)^2
y = (1+200%)^2
y = (1+2) ^2
y = 3^2
y = 9
So, on the fifteenth day, you will have $9
This is an example of exponential growth (y = x(1+r)^t where x = initial starting amount, r = rate of growth expressed as a percent and t = time passed under specific unit). Plugging in your other numbvers ($1000 and $10000) as y will give you the answer to the other problems.
****Note that I may be wrong and it may be $6.25. I'm pretty sure that $9 is right, but for some reason my instinct told me to go with 150% the first time I went through with this. Although 150% is NOT doubled. Please check this with your own work or through someone else.****