Question

On her way home from work, brenda drove 20 miles at 60 miles per hour. due to poor weather conditions, she then reduced her speed to 30 miles per hour for the remainder of the trip. her entire commute to work is 80 miles in distance. based on the information provided above, which numerical value best represents how much less time would it have taken brenda had she driven the entire distance at 40 miles per hour?

Answer 1

Start by finding how far she drove at her reduced speed of 30mi/hr. You know she drove 80mi total, and she already drove 20mi at 60mi/hr. That means she drove 80-20=60mi at 30mi/hr. 

Now you know how long she drove at both speeds, so you can calculate how long it took for her to drive the 80mi because of bad conditions:
$(60mi * \frac{hr}{30mi}) + (20mi * \frac{hr}{60mi}) = 2hr + 1/3 hr = 7/3 hrs$

Now calculate how long it would have taken her to drive in normal conditions at 40mi/hr for 80 miles:
$80mi * \frac{hr}{40mi} = 2 hrs$

Subtract your answer from the second part from your answer in the first: 7/3hrs-2hrs = 1/3 hrs = 20 minutes.

It would have taken Brenda 20 minutes less to drive the entire distance at 40mi/hr.