Question 133573
The variable that is the same for both of them is time. If I
start my stopwatch when they are 725 mi apart and stop it
when they meet, they have both travelled for the same amount
of time.
Let {{{d}}} = distance that Paul travels
{{{725 - d}}} will be the distance that Susan travels
For Paul:
{{{t = d / 75}}} hrs
For Susan:
{{{t = (725 - d) / 70}}} hrs
Since their times are equal,
{{{d/75 = (725 - d) / 70}}}
{{{d/75 = 725/70 - d/70}}}
Multiply both sides by {{{75*70}}}
{{{70d = 75*725 - 75d}}}
{{{145d = 54375}}}
{{{d = 375}}} 
{{{725 - d = 725 - 375}}}
{{{725 - 375 = 350}}}
Now find {{{t}}}
{{{t = d / 75}}}
{{{t = 375 / 75}}}
{{{t = 5}}} hrs
They will meet in 5 hrs
check:
{{{d/75 = (725 - d) / 70}}}
{{{375/75 = 350 / 70}}}
{{{5 = 5}}}
OK