A paint mixture uses blue and white paint in a ratio of 3:5. How many liters of blue paint are needed to make 40 liters of the mixture?

A ratio of $3:5$ means that $3$ of every $8$ parts of the mixture are blue. So the blue paint is $\frac{3}{8}$ of the $40$ liters: $\frac{3}{8}\cdot40=15$ liters, choice G. The white paint is the other $25$ liters, and $15+25=40$. Choice J ($25$) is the amount of white paint. Choice H ($24$) takes $\frac{3}{5}$ of $40$, comparing blue to white rather than blue to the whole. Choice F ($8$) is $\frac{40}{5}$. Blue paint is the smaller part of the mixture, so the answer must be less than $20$.