# Problem solving with Quadratics x3

• Feb 16th 2010, 11:07 PM
Tessarina
Jack and Mack both drive 40 km from home to work each day. One day Jack said to mack, "if you drive home at your usual speed, i will average 40 kmph faster than you and arrive home in 20 minutes less time." Find mack's speed.
• Feb 17th 2010, 04:15 AM
HallsofIvy
Quote:

Originally Posted by Tessarina
Jack and Mack both drive 40 km from home to work each day. One day Jack said to mack, "if you drive home at your usual speed, i will average 40 kmph faster than you and arrive home in 20 minutes less time." Find mack's speed.

Again, V= D/T and T= D/V.

Letting T be Mack's speed and V Mack's time, then T= 40/V. Driving 40 minues faster and 20 min= 1/3 hour less, T- 1/3= 40/(v+40). T= 1/2+ 40/(V+ 40)= 40/V. Solve for V.
• Feb 17th 2010, 11:23 AM
Tessarina
Quote:

Originally Posted by HallsofIvy
Again, V= D/T and T= D/V.

Letting T be Mack's speed and V Mack's time, then T= 40/V. Driving 40 minues faster and 20 min= 1/3 hour less, T- 1/3= 40/(v+40). T= 1/2+ 40/(V+ 40)= 40/V. Solve for V.