15. Rational Expressions and Equations

If $y$ varies directly as $x$, and $y=18$ when $x=6$, find $y$ when $x=10$.

The distance $d$ a car travels varies directly as the time $t$. If the car travels 180 km in 3 hours, find how far it will travel in 5 hours.

$p$ varies inversely as $q$, and $p=18$ when $q=9$. If $q=12$, what is the value of $p$?

The time $t$ (in hours) required to travel a fixed distance varies inversely as the speed $s$ (in m/hr). If it takes 6 hours to travel a certain distance at 50 mi/hr, how long will it take to travel the same distance at 75 mi/hr?