Which of the following sentences is a declarative sentence.

According to the passage a mound is

What does a triple-beam balance require the user to do? add the numbers from the three sliders to determine the mass of an object multiply the numbers from the three sliders to determine the mass of an object add the numbers from the three sliders to determine the volume of an object multiply the numbers from the three sliders to determine the volume of an object

Raul has one solution that is 20% acid by volume and one solution that is 50% acid by volume. he plans to mix together some of each solution to create 30 milliliters of a solution that is 40% acid by volume. how many milliliters of the 20% acid solution should raul use to create the 40% acid solution? can someone explain this to me? i know that the answer is 10 milliliters, but i don't know how to get it.