Samples of ores from the face of a drift on twenty consecutive days and assayed for tungsten (W). The twenty assays average 1.50% W, with a variance of 0.1% W2. Calculate a 90% confidence interval for the tungsten content of the mined mineral. Further mining of the drift would be justified if the grade is above 1.25% W; can we be 95% confident that this is the case?