A car, initially going eastward, rounds a 90 ∘ curve and ends up heading southward.

If the speedometer reading remains constant, what is the direction of the car's average acceleration vector?
Express your answer using three significant figures.

To determine the direction of the car's average acceleration vector, we need to consider the change in the car's velocity over time.

Let's break down the problem step by step:

1. Start by defining the initial and final velocities:
- The car is initially going eastward, so its initial velocity vector can be represented as v_initial = (v_i, 0), where v_i is the initial speed of the car.
- After rounding the 90° curve, the car ends up heading southward. Let's assume its final velocity vector is v_final = (0, v_f), where v_f is the final speed of the car.

2. Calculate the change in velocity:
- Δv = v_final - v_initial
- In this case, Δv = (0, v_f) - (v_i, 0) = (-v_i, v_f)

3. Determine the direction of the change in velocity:
- Since the x-component of Δv is -v_i (negative), it points towards the west.
- Since the y-component of Δv is v_f (positive), it points towards the north.

4. Finally, the average acceleration vector is defined as the change in velocity divided by the time taken. Since the problem states that the speedometer reading remains constant, we can assume the time taken is constant as well. Therefore, the average acceleration vector has the same direction as the change in velocity.

Therefore, the direction of the car's average acceleration vector is northwest.

Please note that the magnitude of the acceleration vector is not given in the problem, so we can't determine it without additional information.