Question

Susie has planned a trip to a city 60 miles away. She wishes to have an average speed of 60 miles/hour for the trip. Due to a traffic jam, however, she only has an average speed of 30 miles/hour for the first 30 miles. How fast does she need to go for the remaining 30 miles so that her average speed is 60 miles/hour for the whole trip?

Answer 1

90 mi/h

Explanation:

Given,

For first 30 miles, her speed is 30 miles per hour,

Let x be her speed in miles per hour for another 30 miles,

Since, here the distance are equal in each interval,

So, the average speed of the entire journey

`=\frac{\text{Average speed for first 30 miles + Average speed for another 30 miles}}{2}`

`=(30+x)/(2)`

According to the question,

`(30+x)/(2)=60`

`30+x=120`

`\implies x = 90`

Hence, she needs to go 90 miles per hour for remaining 30 miles.