Determine the maximum contribution that can be made to a Keogh plan in each of the following cases. In all instances, the individual is self-employed, and the self-employment tax reduction has already been taken.

1. Self-employment income of $51,000.

2. Self-employment income of $51,000 and wage income of $30,000.

3. Self-employment income of $125,000.

4. Self-employment income of $290,000.