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 $69,000.
2. Self-employment income of $69,000 and wage income of $39,000.
3. Self-employment income of $195,000.
4. Self-employment income of $308,000.