A parent established a college fund for his daughter. Each year the parent made a contribution to the fund, and each year he increased his contribution by a constant amount. If he made a contribution of $800 in the first year, by what amount did the parent increase his contribution to the fund each year?
(1) The parent’s contribution to the fund in the 18th year was $7600.
(2) The parent’s contribution to the fund in the 7th year was twice what it was in the 3rd year.
Assuming the C(m) is the contribution according to month m, and the increase amount per month is k. where C(m)=800+(k-1)*m.
(1) C(18)=800+(18-1)*m=7600; Sufficient;
(2) C(7)/C(3)=(800+(7-1)*m)/(800+(3-1)*m)=2; Sufficient.
Therefore, the answer is D.