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 contribution in the 18th year was 7600$
2. The contribution in the 7th year was twice what it was in the 3rd year