Coherent systems are very important in reliability,survival analysis and other life sciences.In this paper,we consider the number of failed components in an (n-k + 1)-out-of-n system,given that at least m (m ＜ k ≤ n) components have failed before time t,and the system is still working at time t.In this case,we compute the probability that there are exactly I working components.First the reliability and several stochastic properties are obtained.Furthermore,we extend the results to general coherent systems with absolutely continuous and exchangeable components.