# Homework Help: I dont understand how to calculate the speed in this problem ?

1. Sep 15, 2008

### papi

Driving along a crowded freeway, you notice that it takes a time t to go from one mile marker to the next. When you increase your speed by 5.0 mi/h, the time to go one mile decreases by 13 s. What was your original speed?
(in mph)

I did R= 1/T
and R= 1/T-13 -5/3600

2. Sep 15, 2008

### smallphi

Make sure it's 1/(T-13). Also, from the way you wrote the equations, your speed R is in miles/second. Maybe that's why the result seems strange to you.

3. Sep 15, 2008

### Redbelly98

Staff Emeritus
Those equations look right to me. What answer did you get for R and T?

Did you check if your numbers were consistent with the two equations?

p.s. Welcome to Physics Forums!

p.p.s. Smallphi is giving good advice here.

4. Sep 15, 2008

### papi

I still dnt get it. Im really bad at physics :(
SO my answer was in miles/ sec opposed to miles/ hr?
i got -103.4651999 miles/ sec then
but how could it be neg, and if it is right, how do i convert it?