Ok, here we have an rule of three problem, so let's do it:
[tex]\frac{420\text{ miles}}{6\text{ hours}}=\frac{x\text{ miles}}{7.5\text{ hours}}[/tex]
We need clear x:
[tex]\frac{420\text{ miles}}{6\text{ hours}}\cdot7.5\text{ hours=x miles}[/tex][tex]525\text{ miles=x}[/tex]
If she maintains the same speed she can go 525 miles further away, so in total counting the miles traveled previously (420), will have traveled a total of 420+525=945 miles.
