PLEASE HELP ASAP!! 50 POINTS!!!

Arnold and Jeremy used two pre-packaged mixes to make lemonade for field day. One of the mixtures contained 20% pure lemon juice and the other contained 70% pure lemon juice. They had hoped to have a final mixture of 20 gallons of 40% pure lemon juice, but when they tasted it, they realized the lemonade was much too tart. Take a look at their calculations and see if you can help them adjust their recipe to get the desired result.

The boys mixed 10 gallons of the 20% pure lemon juice mix and 10 gallons of the 70% pure lemon juice mix, because they wanted 20 gallons of lemonade at a 40% lemon juice mixture. They thought that because 40% was almost halfway between 20% and 70%, they should just mix equal parts of both, but the lemonade turned out too tart. How much of each should they have used to get a final mixture of 20 gallons at 40% lemon juice? Write your answer in complete sentences. Show all work.