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 $46,000.
b) Self-employment income of $46,000 and wage income of $30,000.
c) Self-employment income of $125,000.
d) Self-employment income of $280,000.