From what you said above, it is easy to draw the conclusion that Xiao Ming's mother bought something worth 400 yuan for the first time, and spent 400x0.9 = 360 yuan after the discount, and bought something worth 600 yuan for the second time, and spent 500 x 0.9+100 x 0.8 = 530 yuan after the discount (so 70 yuan was saved compared with her own value 600 yuan); So the total value of shopping is 400+600= 1000 yuan, that is, it took 360+530=890 yuan money to buy something with a total value of 1000 yuan; If you buy something worth 1000 yuan at one time, you have to pay 500 x 0.9+(1000-500) x 0.8 = 850 yuan, so if you buy it at one time, you can save 890-850=40 yuan money compared with the above two purchases.