If the velocity of an object changes from 15 m/s to 50 m/s during a time interval of 4 s, what is the acceleration of the object?

Assuming constant acceleration or that you want average acceleration, you can use the formula:

acceleration = change in velocity / time

=> acceleration = [ 50 m/s - 15 m/s] / 4 s

acceleration = (35 m/s ) / 4s = 8.75 m/s^2

8.75 m/s^2