SOLUTION: David left Boston on the Coast to Coast Bike Ride, riding at 20 km/hr. Forty-five minutes later, Kristi rode out of Boston at 25 km/hr. If these speeds stay the same, how far dow

Algebra ->  Customizable Word Problem Solvers  -> Travel -> SOLUTION: David left Boston on the Coast to Coast Bike Ride, riding at 20 km/hr. Forty-five minutes later, Kristi rode out of Boston at 25 km/hr. If these speeds stay the same, how far dow      Log On

Ad: Over 600 Algebra Word Problems at edhelper.com


    

Question 918367: David left Boston on the Coast to Coast Bike Ride, riding at 20 km/hr. Forty-five minutes later, Kristi rode out of Boston at 25 km/hr. If these speeds stay the same, how far down the road will David be when Kristi catches up to him?
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
David's head start is:
+d%5B1%5D+=+20%2A%28+3%2F4+%29+ ( 45 min = 3/4 hr )
+d%5B1%5D+=+15+ km
--------------------
Start a stopwatch when Kristi leaves
Let +t+ = time on the stopwatch in hrs
when she catches David
------------------------
Let +d+ = distance in km from Boston when
she catches David
---------------------
David's equation:
(1) +d+-+15+=+20t+
Kristi's equation:
(2) +d+=+25t+
---------------
Substitute (2) into (1)
(1) +25t+-+15+=+20t+
(1) +5t+=+15+
(1) +t+=+3+ hrs
and
(2) +d+=+25t+
(2) +d+=+25%2A3+
(2) +d+=+75+ km
----------------
Kristi catches him 75 km from Boston
check:
(1) +d+-+15+=+20t+
(1) +75+-+15+=+20%2A3+
(1) +60+=+60+
OK