Word Problem on distance, time, speed

[Heidiv]

I am having a problem getting started with this word problem:

Rachel allows herself 1 hr to reach a sales appointment 50 mi away. After she has driven 30 mi, she realizes that she must increase her speed by 15mph in order to get there on time. What was her speed for the first 30 mi?

Do I use the formula r=d/t? r=30/t

Thank you!

 

[Mrspi]

I am having a problem getting started with this word problem:

Rachel allows herself 1 hr to reach a sales appointment 50 mi away. After she has driven 30 mi, she realizes that she must increase her speed by 15mph in order to get there on time. What was her speed for the first 30 mi?

Do I use the formula r=d/t? r=30/t

Thank you!

time to drive the first 30 miles + time to drive the remaining 20 miles = 1 hour

So, you will probably want to use this version of the distance-rate-time relationship:

t = d / r

 

[Heidiv]

I am still a little confused. So would that be

t=60 minutes
d= 30 miles
r=x

60=30/r
2=r

t=60
d=20 miles

60=20r
3=r

 

[Mrspi]

I am still a little confused. So would that be

t=60 minutes
d= 30 miles
r=x

60=30/r
2=r

t=60
d=20 miles

60=20r
3=r

No, I'm afraid not.

The speed is in miles per hour, so you want the time to be in hours.

Let r be the rate for the first 30 miles. Then 30/r is the time IN HOURS that it takes to travel that distance.

For the second part of the trip, which is 20 miles, the rate is increased by 15 mph, so the rate for that part of the trip is (r + 15) mph. The time it takes to travel 20 miles at (r + 15) mph is 20 / (r + 15) hours.

30/r + 20/(r + 15) = 1

Now, can you solve this for r?