Here's how to calculate the acceleration of the car:

1. Understand the Concepts

* Acceleration: The rate of change of velocity over time.

* Initial Velocity (v_i): The starting velocity of the car (20 miles/sec).

* Final Velocity (v_f): The velocity after the acceleration (20 miles/sec + 10 miles/sec = 30 miles/sec).

* Time (t): The duration of the acceleration (30 seconds).

2. Formula

The formula for acceleration is:

*acceleration* = (v_f - v_i) / t

3. Calculation

Plug the values into the formula:

*acceleration* = (30 miles/sec - 20 miles/sec) / 30 seconds

*acceleration* = 10 miles/sec / 30 seconds

*acceleration* = 1/3 miles/sec²

Therefore, the acceleration of the car is 1/3 miles per second squared.