To answer this question, we can calculate the time taken for each part of the journey separately and then add them together.
Let's calculate the time taken for the first part of the journey, which is two-thirds of the total distance of 15 miles. The speed for this part is given as 40 mph.
Distance for the first part = 2/3 * 15 miles = 10 miles
Speed for the first part = 40 mph
To calculate the time taken for the first part, we can use the formula:
Time = Distance / Speed
Time taken for the first part = 10 miles / 40 mph = 0.25 hours
Next, let's calculate the time taken for the second part of the journey, which is the remaining one-third of the total distance. The speed for this part is given as 60 mph.
Distance for the second part = 1/3 * 15 miles = 5 miles
Speed for the second part = 60 mph
Time taken for the second part = 5 miles / 60 mph = 0.083 hours
Finally, we can add the times taken for both parts of the journey to get the total time taken:
Total time taken = Time taken for the first part + Time taken for the second part
= 0.25 hours + 0.083 hours
= 0.333 hours
To convert this time to minutes, we multiply by 60:
Total time taken = 0.333 hours * 60 mins/hour = 20 mins
Therefore, the correct answer is D) 20 mins.