Let the rate that Boris can run = R
Let the rate that the escalator moves = r
When he runs up the up escalator his rate = (R + r)
When he runs up the down escalator his rate is (R - r)
And the Distance that he covers (the escalator's length) = L
So..... using Rate * Time = Distance
L = L
(R + r) * 6 = (R - r) *30
6R + 6r = 30R - 30r
36r = 24R
R = (36/24)r
R = (3/2)r → r = (2/3)R
So.......using Distance / Combined Rates = Time
And substituting for r, we have that
L / (R + r) = 6 → L / [ R + (2/3)R] = 6 → L / [ (5/3)R] = 6 →
(3/5)L / R = 6 →
L/ R = (5/3) (6) = 10 seconds to run up (or down) the out of service escalator