Question 920003
t = D/r
r the rate of the truck
400/r - 400/(r+10mph) = 2hr |Multiplying thru by r(r+10)so as all denominators = 1
400(r+10) - 400r = 2r(r+10)
2r^3 + 20r -4000 = 0
r^2 + 10r - 2000= 0  (Tossing out the negative solution for unit measure)
(r + 50)(r -40) = 0
r = {{{ highlight_green(40mph)}}} , the rate of the truck. Car's rate is {{{ highlight_green(50mph)}}} {{{40+10}}}
And...checking
{{{400mi/(40mph) - 400mi/(50mph)}}} = 10hr - 8hr = 2hr