Calculate each of the following sums.

1. $10 + 10.5 + 11 + 11.5 + … + 90$ 

   Hint (a):

   This is an arithmetic series with a common difference of $0.5$.

   More Help:

   $S_n=\frac{n}{2}(a_1+a_2)$
   $a_{1}$ and $a_{n}$ are given in the problem.
   You need to determine $n$, which is the number of terms.

2. $20 + 20(1.05) + 20(1.05)^2 + 20(1.05)^3 + … + 20(1.05)^{20}$ 

3. $400−200+100−50+\dots+6.25$ 

   Hint (b) & (c):

   Parts (b) and (c) are geometric series.
   $S_n=\frac{a(1-r^n)}{1-r}$