Let's start with a "word equation"
Since the travel time is less when the speed is increased, we can write: (travel time at REGULAR speed) = (travel time at INCREASED speed) + 40 minutes

Rewrite as: (travel time at REGULAR speed) = (travel time at INCREASED speed) + 2/3 hours

Let x = the REGULAR speed (in kilometers per hour)
So, x + 40 = the INCREASED speed (in kilometers per hour)

Time = distance/speed
So, after some substitution, our "word equation" becomes: 800/x = 800/(x + 40) + 2/3

ASIDE: At this point, we can either plug each answer choice into the above equation, or we can solve the equation. Let's solve it.
Multiply both sides of the equation by 3 to get: 2400/x = 2400/(x + 40) + 2
Multiply both sides by x to get: 2400 = 2400x/(x + 40) + 2x
Multiply both sides by (x + 40) to get: 2400(x + 40) = 2400x + 2x(x + 40)
Expand: 2400x + 96,000 = 2400x + 2x² + 80x
Rearrange and simplify: 2x² + 80x - 96,000 = 0
Divide both sides by 2 to get: x² + 40x - 48,000 = 0
Factor: (x + 240)(x - 200) = 0
So, x = -240 OR x = 200

Since the speed can't be negative, we can be certain that x = 200

Answer: B