A car leaves a city at 70 km/hr. After 15 minutes, another car follows at 80 km/h. How long will it take for the second car to reach the first, and at what distance?

Answer 1

First, make sure all units are compatible:

Convert 15 minutes to hours, since the speeds are listed in km per hour.

15 minutes / 60 min/hr = .25 hours

Set up algebra equations:
Then let x be the distance traveled by the car that travels at 70 km/hr

Let y be the distance traveled by the car that travels at 80 km/hr

Let t be the time
When the second car starts, the first car has already traveled 70 km/h * .25 h = 17.5 km

x = 17.5 + 70*t
y = 80 * t

The cars will meet when x = y, or when

80*t = 17.5 + 70*t

Solve for t, the time at which they meet. Plug back t into the equation for x or y to find the distance at which they meet.