 |
| ||
| |||
|
| Math Central | Quandaries & Queries |
|
Question from bob, a student: T=(AB)/(A+B) using this formula t= time a,b representing each individual person |
Hi Bob,
This is a question for which I would never use a formula. Formulas are much too easy to forget. For this and many similar problems I just remember to look at rates.
You mow the lawn in 30 minutes so your rate is 1 thirtieth of a lawn per minute (1/30 lawns/min). It takes Joe 3 times as long so it takes him 90 minutes and thus his rate is one ninetieth of a lawn per minute (1/90 lawns/min). Hence if you work together you can work at
1/30 + 1/90 lawns/min,
and 
lawns/minute.
But you want the number of minutes per lawn which is the reciprocal of lawns per minute so it will take you
minutes/lawn.
Exactly what the formula says!
Harley
| ||
| ||
| ||
 |
 |
|
Math Central is supported by the University of Regina and The Pacific Institute for the Mathematical Sciences.