Not more than 200 y=x(0≤x≤200)
More than 200 but less than 400 y = 0.9x+20 (200 < x ≤ 400)
Y = 400 * 0.9+(x-400) * 0.8 = 40+0.8x (400 < x) divided by 400.
( 1)
1The first purchase cost in 30 yuan is lower than that in 200 yuan, so the first purchase cost y = x = 130 yuan.
If the original cost of the second time is 400 yuan, then the actual cost of shopping is 0.9*400+20=380 yuan, and the actual value of the second time is 400 yuan (using the formula of more than 200 but less than 400).
(2)
The sum of the original prices of the goods purchased twice is 400+ 130=530 yuan.
The actual cost is 380+ 130=5 10 yuan.
Then he deposited 530-5 10=20 yuan.
(3)
If the original price of one-time shopping goods in 530 yuan exceeds that in 400 yuan, the formula of more than 400 should be applied.
The actual cost is 40+0.8*530=464 yuan.
According to the first shopping method, he will spend 380+ 130=5 10 yuan.
So two one-time purchases are more economical.
The second method saves 5 10-464=46 yuan than the first one.
This method should be the correct answer. I'm a senior three student, and I haven't done the first-year questions yet. In my opinion, it should be the correct answer. I hope my answer is helpful to you. Thank you.