Problem: The sum of all the integers between 50 and 350 which end in 1, is
Answer Choices:
A. 5880
B. 5539
C. 5208
D. 4877
E. 4566
Solution:
Using the formulas S=21n(a+l) and l=a+(n−1)d for the sum S of n terms and the nth term l where a=51 and d=10, we get
8=51+61+…+341=21n(51+341)=196n where
l=341=51+(n−1)×10.∴n=30.∴S=15×392=5880.