Ascientist has two solutions, which she has labeled solution a and solution b. each contains salt. she knows that solution a is 65% salt and solution b is 80% salt. she wants to obtain 120 ounces of a mixture that is 70% salt. how many ounces of each solution should she use?

Let's make ;
A = ounces solution A
B = ounces solution B

total number of ounces: 
A + B = 120

concentration:
0.65A + 0.80B = 0.7(120)

 Substitute A for 120 - B in the second equation to combine them.

0.65(120 - B) + 0.80B = 84
78 - 0.65B + 0.80B = 84
0.15B = 84 - 78
0.15B = 6
B = 6/0.15
 
B = 40 ounces
Then... A = 120 - 40
A = 60 ounces