Problem: When the sum of the first ten terms of an arithmetic progression is four times the sum of the first five terms, the ratio of the first term to the common difference is:
Answer Choices:
A. 1:2
B. 2:1
C. 1:4
D. 4:1
E. 1:1
Solution:
Using S=2n​[2a+(n−1)d], we have 210​(2a+9d)=4⋅25​(2a+4d)
∴d=2a;∴a:d=1:2.