Let's start by visualising this concept.
Number of grains on square:
1 2 4 8 16 ...
We can see that it starts to form a geometric sequence, with the common ratio being 2.
For the first question, we simply want the fifteenth term, so we just use the nth term geometric form:
![T_n = ar^{n - 1}](https://tex.z-dn.net/?f=T_n%20%3D%20ar%5E%7Bn%20-%201%7D)
![T_{15} = 2^{14} = 16384](https://tex.z-dn.net/?f=T_%7B15%7D%20%3D%202%5E%7B14%7D%20%3D%2016384)
Thus, there are 16, 384 grains on the fifteenth square.
The second question begs the same process, only this time, it's a summation. Using our sum to n terms of geometric sequence, we get:
![S_n = \frac{a(r^{n} - 1)}{r - 1}](https://tex.z-dn.net/?f=S_n%20%3D%20%5Cfrac%7Ba%28r%5E%7Bn%7D%20-%201%29%7D%7Br%20-%201%7D)
![S_{15} = \frac{2^{15} - 1}{2 - 1}](https://tex.z-dn.net/?f=S_%7B15%7D%20%3D%20%5Cfrac%7B2%5E%7B15%7D%20-%201%7D%7B2%20-%201%7D)
![S_{15} = 2^{15} - 1 = 32767](https://tex.z-dn.net/?f=S_%7B15%7D%20%3D%202%5E%7B15%7D%20-%201%20%3D%2032767)
Thus, there are 32, 767 total grains on the first 15 squares, and you should be able to work the rest from here.