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"Fill in your job title. If you are unemployed or a student, put N/A."

Hardly equation solving.

Job Title: ___________ [ ] I am unemployed or a student

Checking the box disables the job title field.

You don't need to make user interfaces that behave like paper forms.

Agreed, as long as the checkbox comes before the field, to avoid wasting time filling in fields that will be ignored anyway.