
Answer:
26 customers
Step-by-step explanation:
First: Â determine the z score from standard normal probability table with an indicative area of 0.1
Z-score from probability table  = - 1.28
mean  = 3.5 minutes
std  = 3 minutes
next determine the Z-score based on the information given in the question
Z = ( std - Â mean ) Â / processing time
 = ( 3 - 3.5 ) / 2 = -0.25
Finally determine the number of customers
N = [tex](\frac{-1.28}{-0.25} )^2[/tex]  = 1.6384 / 0.0625 = 26.21 ≈ 26 customers Â