B. 700
Feedback Let time taken by Sita be Ts seconds to cover 1000m.
For first case, Radha will be covering 900m is (Ts+20)sec (As it is given that Sita takes 20sec less than Radha)
So speed of Radha is (900/Ts+20) m/s
For second case, Radha will take Ts second to cover 875m (1000-125)
so speed of Radha is (875/Ts) m/s
Equating both speed of Radha we get 900Ts = 875(Ts+20) => Ts=700 sec.