In mixture problems you need to keep track of two quantities: The total quantity of mix, and the quantity of the component of interest.
Example
In mixed nut problems, you need to track the total nut weight (peanuts + cashews), and the weight of the nut of interest (cashews).
_____
You won't increase the percentage of the item of interest by diluting it with a mix having a smaller percentage than what you're starting with. Mixing a 30% solution and a 50% solution will not get you a 70% solution. Rather it will get you something between 30% and 50%.
Example
In mixed nut problems, you need to track the total nut weight (peanuts + cashews), and the weight of the nut of interest (cashews).
_____
You won't increase the percentage of the item of interest by diluting it with a mix having a smaller percentage than what you're starting with. Mixing a 30% solution and a 50% solution will not get you a 70% solution. Rather it will get you something between 30% and 50%.