(100p-4200)/100p=30%要演算过程

(100p-4200)/100p=30%
100p-4200=30p
70p=4200
p=60

验算，当p=60时
(100*60-4200)=1800
100*60=6000
1800/6000=30%
等式成立，结果正确，p=60

(100p-4200)/100p=30%
1-42/p=0.3
两边同乘以p得p-42=0.3p,
所以0.7p=42，p=60

(100p - 4200 )/100p=30%

解 :

100p - 4200 =100p * (3/10) =30P

即有 70p = 4200
p=60