Bunuel wrote:

[Deb normally drives to work in 45 minutes at an average speed of 40 miles per hour.

45 minutes = \(\frac{45}{60}\)=>3/4 hours

Distance = Speed x Time

Total distance from office to home is 40x\(\frac{3}{4}\)=>30 miles

Bunuel wrote:

This week, however, she plans to bike to work along a route that decreases the total distance she usually travels when driving by 20% .

Effective distance = 30x\(\frac{80}{100}\)=>24 miles.

Bunuel wrote:

If Deb averages between 12 and 16 miles per hour when biking, how many minutes earlier will she need to leave in the morning in order to ensure she arrives at work at the same time as when she drives?

Let speed be 12 miles/hour ( Minimum speed )

As we know , \(Time\) \(taken\)= \(\frac{Distance}{Speed}\)

Time required = 24/12=>2 hours ( Maximum Time )

Let speed be 16 miles/hour ( Maximum speed )

Time required = 24/16=>\(1\) \(\frac{1}{2}\) hours ( Minimum time)

Since the speed varies from 12 to 16 miles/hour and so does time from 2 hours to 1.5 hours , we must take maximum time taken for claculating the difference.

robu robu wrote:

why the speeds such as 15 ,16 not considered. please explain. thankyou

We are not taking "15" because this problem boils down to a maximum / minimum missue....

The more your speed = The less time you take ; The less your speed = The more time you take

We can say Time taken \(α\) \(\frac{1}{speed}\) { Speed is inversely proportional to time }

I have taken 16 , just to show here that time taken decreases from 2 hours to 1.5 hours.However our problem requires us t find out

Bunuel wrote:

how many minutes earlier will she need to leave in the morning in order to ensure she arrives at work at the same time as when she drives?

The issue is pretty similar to our daily chores in life

While going for office we always take the maximium time required and plan our journey accordingly to avoid any uncertain incident while reaching office.

So, we take the minimum speed, here and consider 12.Rest I think is clear for you , plz feel free to revert in case of any doubt, I will be more than happy to help you.

_________________

Thanks and Regards

Abhishek....

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