An employee at a health food company is making a new type of trail mix. She starts with 10 lb of Hearty Mix, which contains 25% dried fruit by weight. She then adds x lb of Active Mix, which contains 40% dried fruit by weight. Let y be the percent of dried fruit in the new type of trail mix.Which function models situation?

The correct answer is the third choice.   25% of the 10 pounds is 2.5 pounds and to represent 40% of x pounds would be represented as 0.4x.  Together this is the amount that is dried fruit.  

This gets placed over the total weight of mix (10 + x), and then multiplied by 100 to create the percent that is represented.