Find the sum of the integers between 301 and 400 inclusive that are multiples of 4.

Short Answer 8800

Step One
Find the first term after 300 that is divisible by 4.
300 / 4 = 75. It is divisible by 4, but it is not in the series of numbers. The next number that is divisible by 4 is 304
304 / 4 =76 

Step Two
Solve for the number of terms in the series.

Givens
a = 304
L = 400
n = ??
d = 4

Formula
L = a + (n - 1)*d

Substitute and solve
400 = 304 + (n -1)*4   Subtract 304 from both sides.
400 - 304 = (n - 1)*4
96 = (n - 1)*4              Divide by 4
96/4 = (n-1)  
24 = n - 1                    Add 1 to both sides.
24 + 1 = n
n = 25

Step 3
Find the sum

Formula
Sum = (a + L )*n/2

Givens
a = 304
L = 400
n = 25

Sub and solve
Sum = (304 + 400)*25/2
Sum = 704*25/2
Sum = 8800