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:


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:



Thus, there are 32, 767 total grains on the first 15 squares, and you should be able to work the rest from here.
D
If your pH is lower, it has more Hydronium ions
As a rule, for every decrease in pH, there is 10 times more H3O+ so since it is 4.1 there should be a bit more than 10^4. So it has to be D
All the opposite angles are equal in a parallelogram:
so AB=DB shows it is an isosceles triangle:
so the two angles on the sides are congruent,
let x to be one of the angles:
2x+28=180
2x=152
x=76=∠A
∠A is opposite of ∠DCB so ∠DCB is 76° because of the parallelogram properties