Lets say the rate of the plane is x then the rate of the car is x-80.
We know their times are equal so you will solve for time for each.
The time for the plane is
The distance the plane traveled is 260 miles. The distance the car traveled is 100.
The time for both are equal so the distance/rate of the plane = distance/rate of the car
We said the rate for the plane was x and the rate of the car was x-80. Now we just plug in
d/r(plane) = d/r (car) Ignore the parentheses except to differentiate what they are.
260/x = 100/(x-80)
you will cross multiply to get
260x - 20800= 100x
Subtract 260x to take it to the other side and you will get
x = 130 The speed of the plane.