Third person singular present tense: subtracts money or value from a total, or uses logical reasoning to reach a conclusion.
Present tense third-person singular of 'deduct,' from Latin 'deducere' (to lead down/away). This is the form used when the subject is a single person, organization, or thing performing the action.
The IRS 'deducts' taxes before you even see your paycheck, and mathematicians 'deduct' numbers from equations—these are literally the same action separated by 2,000 years and two different fields of human thought.
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