Statistics and probability

at - w) (5.2) Let X, denote the water level in a dam at time t, where t is measured in months. We will assume that at least until the first time that the dam gets empty (i.e. X, = 0), X can be modeled as a Brownian motion with drift / = +1 and variance o = 1. if the initial water level is Xo = 200, find the probability that the water level reaches 210 before dropping below 100