^{We would like to measure the weight of four subjects Y1, ..., Y4 by using a scale four times. We know that there is a error of the scale. Suppose that the error follows a normal distribution with mean 0 and variance ^2 . Errors on different weighings are independent. Please design this experiment; give the estimators of these four subjects and the distribution of these estimators. You should try to minimize the variances of these estimators.}