6. (20%) Throwing two fair dice, each has six faces (1,2,3,4,5,6 points). $X1, X2$ represent the number of results respectively. Y=(X1, X2). Please calculate: (1) (10%)E(Y)

7. (15%) There are N students in a class. How large N will let the probability of at least two students having the same birthday exceed 90%? Note: A year has 365 days. 【Hint: You may use the following characters: (1) $e^{-x}\geq 1 - x$ (2) $e^{-1} = 0.3678;\ln (0.1) = -2.30259$