When travelling with a headwind, the overall speed travelled is s – headwind, so in this case, the overall speed would be s – 15. We use this speed and multiply it by the time travelled (2hr 20min = 2 1/3hr = 7/3hr) to determine the distance. In other words:

d = (s – 15)*(7/3) = (7/3)s – 35

For the trip back, the wind was behind the plane, so the overall speed is s + 15. We use this speed and multiply it by the time travelled (1hr 24min = 1 2/5hr = 7/5hr) to determine the distance. In other words:

d = (s + 15)*(7/5) = (7/5)s + 21

Set both equations equal to each other:

(7/3)s – 35 = (7/5)s + 21

Add 35 to both sides

(7/3)s = (7/5)s + 56

Subtract (7/5)s from both sides

(7/3)s – (7/5)s = 56

Find the least common denominator for both fractions

(35/15)s – (21/15)s = 56

Simplify

(14/15)s = 56

Multiply both sides by 15/14

s = 60km/h

Plug your speed into one of the first equations and solve for d:

d = (7/3)*60 – 35

d = 105km