A plane flies 450 miles with the wind and 300 miles against the wind in the same length of time. If the speed of the wind is 20mph, what's the speed of the plane in still air?

a) 90mph
b) 105mph
c) 100mph
d) 125mph

Is there a formula to solve this? I am just a student pilot, but it seens to me like I would need the time and heading to solve this!? Am I wrong?
Thank you guys.