Fie x pretul stiloului
x·(100-q)/100·(100+p)100=x /÷x
(100-q)(100+p)=10000
D₁₀₀₀₀={1,2,4,5,8,10,16,20,25,40,50,80,100,125,200,250,400,500,....10000} ⇒
10000= 80×125 ⇒ 1000= (100-20)×(100+25)⇒ q=20% si p=25%, acestea fiind singurele solutii , in conditiile in care p,q<100