How many liters of a 40% acid solution must be mixed with 10 liters of a 10% acid solution to create a mixture that is 25% acid?

Let $x$ be the liters of $40\%$ solution. The acid in the mixture comes from both solutions: $0.40x+0.10(10)=0.25(x+10)$. This becomes $0.40x+1=0.25x+2.5$, so $0.15x=1.5$ and $x=10$ liters, choice C. Check: $4+1=5$ liters of acid in $20$ liters of solution, which is $25\%$. Choice A ($5$ liters) gives $3$ liters of acid in $15$ liters, which is $20\%$. Choice B ($8$ liters) gives $4.2$ in $18$, about $23\%$. Choice D ($15$ liters) gives $7$ in $25$, which is $28\%$. Only $10$ liters reaches exactly $25\%$.