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.

a. Self-employment income of $51,000

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

c. Self-employment income of $135,000

d. Self-employment income of $315,000