Average speed$= \frac{total distance travelled}{ total travel time}$

The driver travels a total distance of 40 miles, and were told that the average speed was 60 miles per hour.
Plug in those values to get: $60= \frac{40}{ total travel time}$

Now let's calculate total travel time
total travel time = (time spent driving the first 20 miles) + (time spent driving the last 20 miles)

Since $time = \frac{distance}{speed}$, we get:
total travel time $= \frac{20}{50} + \frac{20}{x}$

total travel time $= \frac{20x}{50x} + \frac{1000}{50x}$

total travel time $= \frac{20x+1000}{50x}$

Plug this into our equation: $60= \frac{40}{(\frac{20x+1000}{50x})}$

Rewrite as follows: $60= (40)(\frac{50x}{20x+1000})$

Simplify: $60= \frac{2000x}{20x+1000}$

Multiply both sides of the equation by $20x+1000$ to get: $1200x +60,000 = 2000x$

Subtract $1200x$ from both sides to get: $60,000 = 800x$

Divide both sides by $800$ to get: $75 = x$

So we have:
QUANTITY A: $x - 60 = 75 - 60 = 15$
QUANTITY B: $10$

Answer: A