Brand X sells 21 oz. bags of mixed nuts that contain 29% peanuts. To make their product they combine Brand A mixed nuts which contain 35% peanuts and Brand B mixed nuts which contain 25% peanuts. How much of each do they need to use?
A detailed explaination of how to get the answer would be greatly appreciated thanks!
As noted in a comment, you had already found that
$$(0.29)21= (0.35)A + (0.25)B.$$
The interpretation of this is that $A$ is the amount of mixed nuts of Brand A that are used, and $B$ is the amount of mixed nuts of Brand B. The left side indicates that the mixture contains $21$ ounces of nuts.
To be extra clear, I'll repeat the preceding paragraph in slightly different words:
The manufacturer takes $A$ ounces of one brand of nuts and $B$ ounces of another brand of nuts to make $21$ ounces of mixed nuts.
Using just the information in the previous paragraph, you can write another equation involving $A$ and $B$. The numbers $0.29$, $0.35$, and $0.25$ were not mentioned in that paragraph, so they do not occur in the equation.
You will then have a sufficient mathematical model to solve the problem.