Final answer:
To calculate the speed of a boat that sails 20 km in half an hour, multiply the distance by 2 to adjust for one hour, yielding a speed of 40 km/h.
Step-by-step explanation:
If a boat sails 20 km in half an hour, the calculation of its speed in kilometers per hour is straightforward. Since speed is equal to distance divided by time, we simply need to adjust the half-hour time period to one hour to find the boat's speed for one hour. To do this, we multiply the distance (20 km) by 2, since half an hour is half of one hour, and 1 hour is the standard unit for measuring speed in km/h.
Therefore, the speed of the boat is 20 km × 2 = 40 km/h.
This means the correct answer is B) 40 km/h.