Let r be the speed of the plane, in mph.

Then the speed of the car is (r-100) mph.


You then have this "time" equation

 =         ("A plane can fly 300 miles in the same time as it takes a car to go 100 miles").


To solve it, cross-multiply. You will get

300*(r-100) = 100*r  ====>  300r - 30000 = 100r  ====>  200r = 30000  ====>  r =  = 150.


Thus the plane speed is 150 miles per hour.


Then the car' speed is 150-100 = 50 mph.