Answer:
A. 10.2 days.
Step-by-step explanation:
Production rate(p) = 100 per day 
Demand rate(d) = 20 per day 
Annual demand(D) = 5000 units 
Set up cost(S) = $40 
Monthly Holding cost = $1 . So annual holding cost (H) = $1*12 = $12 per unit
Optimum run size(Q) = √{2DS / H [1-(d/p)]} 
= √{(2*5000*40) / 12*[1 - (20/100)]} 
= √[400000/12*(1-0.20)] 
= √ [400000/(12*0.80)] 
= √(400000/9.6) 
= √41666.66666 
= 204.12
Cycle length = Q/d
Cycle length = 204/20
Cycle length = 10.2 days