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