Question 1055819
He walks 1.5 m/sec
The walkway has x m/sec speed
280/(1.5+x) is the time it takes him to do it while walking.  The units are seconds.
That is 60 seconds fewer than 280/x, the time it takes him without walking
Therefore 280/(1.5+x)+60=(280/x), meaning that the time by walking is 60 seconds less than by standing.
(280 +90+60x)/(1.5+x)=(280/x)
or (370+60x)/(1.5+x)=(280/x)
cross-multiply
420+280x=370x+60x^2
60x^2+90x-420=0; now, divide by 30
2x^2+3x-14=0
(2x+7   ) (x -2   )=0
x= 2 m/sec, only positive root.  ANSWER
Check
It takes him 140 sec to go completely assisted. 280 m/2 m/sec
If he walks, he is going 3.5 m/sec, combined speed
That takes 280/3.5 or 80 seconds to go 280 meters.