Question $2(2+2+2+2+2=10$ points$)$ Normal demand for newsvendor model

Consider a clothing item satisfying assumptions of the newsvendor model:

Demand ~ Normal $[\backslash$mu $=100,\backslash$sigma $=20].$

The wholesale cost is $$20/$unit$.$ For every unit sold, there is a profit of $$10$ per unit $($Revenue $=$ cost $+$ profit $=20+10=$ $$30).$ Unsold units are disposed. For every leftover unit, there is a loss of $$5/$unit$.($Loss $=$ cost $$ salvage value$)$

a$.)$ What is the mean demand?

b$.)$ What is the newsvendor critical ratio?