一桶油 第一次取出全桶油的 五分之一第二次取出20千克 第三次取出的等于前两次取出的总和,这时还剩下8千克

设原来有油x千克.
x=（1/5x+20）×2+8·································理条件,总油=（第一次+第二次）×2【第三次】
x=2/5x+40+8············································运用乘法分配律拆括号
3/5x=48·······················································两边同时减2/5x,并将40、8合成一体
x=80························································x=48÷3/5,解出方程
答：原来这桶油有80千克.

设原来这桶油有x千克
根据题意得：[(1/5)x+20]*2=x
解得：x=200/3
答：原来这桶油有200/3千克.