A car traveling westward at 20 m/s turns around and travels eastward at 15 m/s. If this takes place in 5 s, what is the average acceleration of the car?
15-20= -5= -1
5 5
Is this worked correctly
No, you have an error with your signs.
Let's say the eastward direction is the positive direction. Let's demand units of meters and seconds everywhere.
Then the problem says $\displaystyle v_i = -20, v_f = 15$ at times $\displaystyle t_i = 0, t_f = 5$.
Thus the average acceleration over those 5 seconds is given by:
$\displaystyle a_{ave} = \frac{v_f - v_i}{t_f - t_i} = \frac{(15) - (-20)}{(5) - (0)}= \frac{15 + 20}{5}= \frac{35}{5} = 7$.
Thus the average acceleration over those 5 seconds was 7 meters per second squared.