A9 l solution that was 11% vinegar was mixed with a 10 l solution that was 69%vinegar. find the new concentration of vinegar

1st solution:    9L      11% Vinegar,     9,    11
2nd solution:  10L      69% Vinegar    10,  69
Total volume when mixed = 9 + 10 = 19

Using, law of mixtures:

Volume1 * Concentration 1 +  Volume2 * Concentration2 =   Total Volume * Final Concentration
Volume is in L, and Concentration is in %

9*11 + 10*69 = 19* FC,    Where FC = Final Concentration
99 + 690 = 19FC
789 = 19FC     Divide both sides by 19

789/19  = FC
41.52  =  FC

Therefore final concentration after mixing =  41.52%

The first answer is correct

She will need 5/8 bags for each square yard.