The time that the express takes is t_e = 250/e, and the time the local train takes is t_s = 250/s. As s = e - 10 we are left with:
t_e = 250/e,
t_s = 250/(e-10).
We are also told that:
t_e = t_s - 1.25,
so we have:
250/e = 250/(e-10) - 1.25.
Multiply through by e(e-10) to get:
250 (e-10) = 250 e - 1.25 e (e-10)
which simplifies to the quadratic:
1.25 e^2 - 12.5 e - 2500 = 0,
which has roots e=50 and e=-40, but as e should be positive we find