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PS:一种车用的液体里面有水和Brand X的一种产品(这个产品本身不含水)一共9 Litter,现在这个液体的水含量是75%,求问这个液体要drain out多少之后再加入brand x产品能达到50%含水量?
Not sure if this is correct. Open to discussion.
1. Brand X in the product is 9 * (1-75%) = 9/4 L = 2.25 L
2. Assume removing Y litter of product and add brand X.
3. Removing Y litter of product will also remove Y*0.25 Brand X from the product.
4. Since we are adding brand X into the product after the removal of Y litter of product, total amount remains the same. so
(2.25 - Y*0.25 + Y) / 9 = 50%
X = 3
Now, put it back into the question to validate.
Since we are going to remove 3 litter of product, we are also removing 3*0.25 = 0.75 L of brand X.
So, after removing 3 litters, there will be 2.25 - 0.75 litters of brand X left. Adding 3 litters of brand X into the product will make the brand X = 4.5
Total amount of product remains the same. So, 4.5 / 9 = 50%.
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