Question 407634
"The rate of the express is 20 miles per hour faster than the rate of the freight train"
___ e = f + 20


t = d / r ___ the freight train time is 3 hr longer


(200 / f) - 3 = 120 / e


substituting ___ (200 / f) - 3 = [120 / (f + 20)]


multiplying by [f(f+20)] ___ 200(f+20) - 3[f(f+20)] = 120(f)


200f + 4000 - 3f^2 - 60f = 120f ___ 0 = 3f^2 - 20f - 4000


factoring ___ 0 = (3f + 100)(f - 40)


3f + 100 = 0 ___ f = -100/3 ___ negative value not realistic


f - 40 = 0 ___ f = 40 ___ substitute back to find e