Question 791346
<font face="Times New Roman" size="+2">
If A can do a job in <i>x</i> time periods, then A can do *[tex \Large \frac{1}{x}] of the job in 1 time period.  Likewise, if B can do the same job in <i>y</i> time periods, then B can do *[tex \Large \frac{1}{y}] of the job in 1 time period.


So, working together, they can do


*[tex \LARGE \ \ \ \ \ \ \ \ \ \  \frac{1}{x}\ +\ \frac{1}{y}\ =\ \frac{x\ +\ y}{xy} ]


of the job in 1 time period.


Therefore, they can do the whole job in:


*[tex \LARGE \ \ \ \ \ \ \ \ \ \  \frac{1}{\frac{x\ +\ y}{xy}}\ =\ \frac{xy}{x\ +\ y}]


time periods.


====================================


The first train has a 7 hour head start.  So how far did it go in those 7 hours a t 75 miles per hour?  Subtract that amount from 1900 miles.  After that, the rate at which they are moving apart, since they are going in opposite directions, is the sum of the two trains' speeds.  Add the two speeds and divide into the remaining distance you just calculated.  You can express it as a number with a decimal fraction, or a mixed number, or as hours and minutes.  Hint if you want to do hours and minutes:  One-fifth of an hour is 12 minutes.


John
*[tex \LARGE e^{i\pi}\ +\ 1\ =\ 0]
<font face="Math1" size="+2">Egw to Beta kai to Sigma</font>
My calculator said it, I believe it, that settles it
<div style="text-align:center"><a href="http://outcampaign.org/" target="_blank"><img src="http://cdn.cloudfiles.mosso.com/c116811/scarlet_A.png" border="0" alt="The Out Campaign: Scarlet Letter of Atheism" width="143" height="122" /></a></div>
</font>