lets assume that the plane is flying with constant speed v

v = the distance / the time

let d = distance and t = 8 1/2 = 17/2 hours the length of the flight

v = d / (17/2) = 2d/17 miles/hours

if we have passed 1/4 of the way that is d/4 and t2 is the time passed we have

v = (d/4) / t2 = d / (4*t2) miles/hour

now we can write

2d/17 = d/(4*t2) divide both sides by d

2/17 = 1/(4 *t2)

t2 = 17/8 hours = 2.125 hours

one forth of the trip takes 2.125 hours.