The question asks for the cyclist's average speed over the entire trip.

1. Distance traveled:

   • First part: 20 km
   • Second part: 40 km
   • Total distance = $20 + 40 = 60$ km

2. Time taken:

   • First part: 1 hour 40 minutes = 1.67 hours (since $\frac{40}{60} \approx 0.67$)
   • Second part: 2 hours
   • Total time = $1.67 + 2 = 3.67$ hours

3. Average speed: Average speed is given by the formula: $\text{Average speed} = \frac{\text{Total distance}}{\text{Total time}}$ $\text{Average speed} = \frac{60 \, \text{km}}{3.67 \, \text{hours}} \approx 16.35 \, \text{km/h}$

Thus, the closest answer option is B. 15 km/h.

Would you like further details or clarification on the steps? Here are some related questions you might want to explore:

1. How do you convert minutes to hours?
2. What is the general formula for calculating average speed?
3. How would the result change if the cyclist traveled a different distance in the same amount of time?
4. Why is the average speed not simply the average of the two speeds?
5. What happens to the average speed if the total time decreases?

Tip: Always ensure that all units (like time and distance) are consistent before performing any calculations.