Question 360058
How far will your have gone? She left 3/4 of an hour before you and will drive a total of 3/4 + 4 = 19/4 hours.
At 60mph, she will travel {{{Distance = Speed * Time}}}
{{{Distance = 60 * 19/4}}}
{{{Distance = 285}}} miles

So the total distance is 285.
How fast do you have to drive to cover 285 miles in just 4 hours?
{{{Distance = Speed * Time}}}
{{{285 = Speed * 4}}}
{{{285/4 = Speed}}}
{{{71.25}}}mph 
Be careful of the speed limits! A few extra minutes is better than a speeding ticket!!