Question 144742
d = rt   let x be the speed on the last part of the trip

100 = (r+20)t[1]
t[1] = 100/(r+20)


80 = rt[2]
t[2] = 80/r

t[1] + t[2] = 4
{{{(100/(r+20)) + 80/r = 4 }}}

{{{((100 r) + 80(r+20))/(r * (r+20)) = 4 }}}

I bet you can take it from here.

Let me know if you can't