If you mix 1 liter of the 10% solution with 1 liter of the 30% solution, you will have 2 liters of 20% solution. Keeping that in mind, you must mix half of the number of liters with 10%, and the other half with 30% in order to get 20%. So knowing all those things, we can do the problem. She needs 15 liters of 20%, so you need to mix 7.5 liters of 10% with 7.5 liters of 30%. After mixing those, you'll have 15 liters of 20% solution. I hope this helps. Please put as brainliest.