A restaurant uses an average of $50$ jars of a special sauce each week. Weekly usage of sauce has a standard deviation of $3$ jars. The manager is willing to accept no more than a $10$ percent risk of stock during lead time, which is two weeks. Assume the distribution of usage is normal.

a$.$ Which of the given formulas is appropriate for this situation? Why?

b$.$ Determine the value of z$.$

c$.$ Determine the reorder point $($ROP$)$