Would really appreciate if someone could help !!!

1) If a large number of samples of size n are taken from B(20,0.2) and approximately 90% of the sample means are less than 4.354, estimate n.

2) i. The random variable $\displaystyle X$ has the distribution $\displaystyle N(50, 8^2)$. Given that $\displaystyle X_{1} $ and $\displaystyle X_{2}$ are to independent observations of $\displaystyle X $ , find $\displaystyle P( X_{1} > X_{2} + 15) $

ii. The random variable $\displaystyle Y $ is related to $\displaystyle X $ by the formula $\displaystyle Y = aX + b $, where $\displaystyle a $ and $\displaystyle b $ are constants with $\displaystyle a > 0 $. Given that $\displaystyle P(Y < 74) = P(Y > 146) = 0.00668 $, find the values of $\displaystyle E(Y) $ and $\displaystyle Var(Y) $ , and hence find the values of a and b.

thanks!!