After reading Robert's answer, I would guess that the correct equation is:
t(v) = 253/(v+70) + 413/(v+75)
Then you drove 253 miles where the speed limit was 70 mph and 413 miles where the speed limit was 75.
Then t(2) = 253/72 + 413/77 = 253/72 + 59/11.
t(2) = (216 + 37)/72 + (55 + 4)/11
t(2) = 3 + 37/72 + 5 + 4/11
t(2) = 8 + 37/72 + 4/11
t(2) ≈ 8 + 0.5 + 0.4 ≈ 8.9 hours
How close is our estimate?
t(2) = 8 + 37/72 + 4/11
t(2) ≈ 8 + 0.51388888888889 + 0.36363636363636 ≈ 8.87752525252525 hours
Darn close!
And the estimate did not use a calculator (in case you forgot it, it's broken, the batteries are dead, you lent it to a friend, the dog chewed it up, it's a new whizbang graphing calculator your parent's bought you for your birthday and you don't know how to use it because you didn't read the user's manual and do the practice problems, etc.)
2/21/2014

Steve S.