Let's use the DST formula
speed = distance / time
Let's take distance as d and time as t (t1 for general scenario, t2 when speed doubled)
Regular situation Todd's speed is 40miles per hour
40 = d / t1
d = 40*t1 ------------ 1

Now when the speed doubles the time (t1) is reduced by 16mins. New speed is 80 miles per hour
Convert 16mins into hours (16/60)
80 = d / (t1 - (16/60)) ----------- 2

substitute 1 in 2

80 = (40*t1) / (t1 - (16/60))
solving this we get
t1 = 8/15 hours

substitute this in 1
d = 40 * 8/15
d = 64/3

Hence Option D