book 4 page 171 4th ed

[walid_afram]

liam is pulled over for speeding just as he is arriving at work. He explains to
the police officer that he could not afford to be late today, and has arrived at
work only four minutes before he is to start. The officer explains that if liam
had driven 5 mph slower for his whole commute, he would have arrived at
work exactly on time. If liam's commute is 30 miles long, how fast was he
actually driving? (Assume that liam drove at a constant speed for the duration
of his commute.)



Could you please explain to me what's wrong with my following approach:

30 = (r - 5)/60 x (30*60/r + 4)

[messi10]

Please explain what you are trying to do. What is r here and are you trying to use D = R x T formula?

[walid_afram]

Actually there's nothing wrong with the formula, i tried it again and the answer is correct:

r is the speed at which he actually drove

(r - 5) is the speed IF he drove 5mph slower
THEN he'd need 4 more minutes; (30*60/r + 4)

multiplying and dividing by 60 are for converting hours to minutes

PS: whatever the speed is, the distance is the same

hope this helps
Cheers

[tim]

:)