What is the average velocity in the time interval 3 to 4 seconds

This is for my 8th grade science class, i really don’t understand. can someone me? , lol.

When a vertical beam of light passes through a transparent medium, the rate at which its intensity i decreases is proportional to i(t), where t represents the thickness of the medium (in feet). in clear seawater, the intensity 3 feet below the surface is 25% of the initial intensity i0 of the incident beam. what is the intensity of the beam "10" feet below the surface? (give your answer in terms of i0. round any constants or coefficients to five decimal places.)

Multiply or divide to make this english distance conversion. 174 inches = feet (14.5, 58, 2,088, 23)