Answer:
2 gallons
Explanation:
Let g represent the number of gallons of 100% lemon juice that needs to be in the mix. Then the total amount of lemon juice is ...
1.00x + 0.30(7 -x) = 0.50(7)
0.7x +2.1 = 3.5 . . . . . . eliminate parentheses, collect terms
0.7x = 1.4 . . . . . . . . . . subtract 2.1
x = 2 . . . . . . . . . . . . . . divide by 0.7
2 gallons of 100% lemon juice should be added to the mixture to make 7 gallons of 50% lemon juice.