Z-score Stats question

Question

I have no idea evn how to start this question 

The average Demand for renatl skis on winter Saturdays at a particular area is 150 pairs which has been quites stable over time. There is variation due to weather conditions and competing areas; the standard deviation is 20 pairs. the demand distribution seems to be roughly normal.

A. the rental shop stocks 170 pairs of skis. What is the probability that demand will exceed this supply on any winter saturday? B. How many pairs of skis in stock does the shop have to have to make the probability in question A less than .01?

Answer 1

A. z= (x-µ)/σ = (170-150)/20 =1

From the z-table, we can get the upper tail p vaule = 0.1587

so 15.87% will exceed.

 

B. Z upper tail p value <0.1 has Zscore 1.28

So 1.28 = (x-150)/20

x=175.6