10% of the 12 lb mix is banana chips, which comes out to 1.2 lb of banana chips. If [tex]b[/tex] is the amount of banana chips the manager will add to the mix, then the new mix's total weight would be [tex]12+b[/tex], and the manager wants 25% of this to consist of banana chips. This means the final amount of banana chips, [tex]1.2+b[/tex], should come out to 25% of [tex]12+b[/tex]. In other words,
[tex]\dfrac{1.2+b}{12+b}=0.25\implies b=2.4[/tex]
so the manager should add 2.4 lb of banana chips.
