Question 35103
Sum S(n) = n/2(a1+an)
(where n = number of terms; a1 = first term; an = n-th term)
a1 = 5, an = 3, n = 20 
=> S(20) = 10(8) = 80; 
Try doing the rest as above;
If d is known instead of an, Sum S(n) = n[a1 + (n-1)d/2]
You can use one of the above formula to find the sum of all listed problems;
For last two problems, you will have to use: an = a1+(n-1)d too.