Question 172019
A)  
Letx=amount of 4% brine solution needed

Now we know that the amount of pure salt in the 4% solution that is added(0.04x) plus the amount of pure salt in the 12% solution(0.12*40) has to equal the amount  of pure salt in the final mixture(0.08(40+x)), so our equation to solve is:

0.04x+0.12*40=0.08(40+x)  get rid of parens (distributive)
0.04x+4.8=3.2+0.08x subtract 3.2 and also 0.04x from each side
0.04x-0.04x+4.8-3.2=3.2-3.2-0.04x+0.08x  collect like terms
1.6=0.04x  divide each side by 0.04
x=40 liters--------------------amount of 4% brine solution needed

CK
0.04*40+0.12*40=0.08*80
1.6+4.8=6.4
6.4=6.4

B)  
I assume an average score of 80 is a B??????
Let x=score that she must make on the last test
Then (85+78+73+x)/4=80  multiply each side by 4
85+78+73+x=320 or
236+x=320  subtract 236 from each side
x=320-236=84----------------------------this would give her an average of 80

CK
(85+78+73+84)/4=320/4=80




Hope this helps---ptaylor