Let $\theta$ be the mean of the sample of Size n. when, $Z = (\theta - \mu) * \sqrt{n} / \sigma$ then, Z ~ N(0,1). (Please let me know if you need the proof) Now, for the sample mean to be > 0.25, we can use the standard normal distribution of Z, The probability can be given by P(Z > 0.07418), which is -$P(Z>0.07418) = 1 - \phi (0.07418)$ Using $\phi (0.07) = 0.52790$ and $\phi (0.08) = 0.53188$ and using the method of Interpolation - $\phi (0.07418) = 0.5296$ Hence the Desired probability is 1 - 0.5296 = 0.4704