Which is a linear function? 1.
The population of a town increases by 2% every 5 years.
The value of a $45,000 car depreciates 15% of its value each year.
2.
3
A radioactive substance decays by a factor of one-eighth each day.
4.
A gym charges a one-time membership fee plus a monthly fee of $19.99.
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4. A gym charges a one-time membership fee plus a monthly fee of $19.99.

Step-by-step explanation:

Linear functions increase or decrease by the same amount at each interval.

For example, as the x value increases by 1, the y value increases by 5.

So, option 4 is correct, because the fee is the same each month.

The other answer choices are not linear because they increase or decrease by percentage values.