The first solution contributes 0.78*1 = 0.78 L of acid.
The second contributes 0.64*6 = 3.84 L of acid.
There will be 7 L total of the new solution, and 3.84 + 0.78 = 4.62 total L of acid in it. So the new solution has a concentration of 4.62/7 = 0.66, or about 66%.
She used 3/8 of the cheese. Since she used 1/2 <em>of</em> 3/4, you multiply 3/4 times 1/2. If you multiply straight across, the answer should be 3/8. If you need further explanation, just ask! Hope this helps! =D :D
