Question 634557
Let x=amt of pure silver needed
Now we know that the amount of PURE silver that exists before the mixture takes place has to equal the amount of PURE silver that exists after the mixture takes place:

Pure silver before the mixture takes place: x+180*0.75=x+135
Pure silver after the mixture takes place: (180+x)*0.80=144+0.80x

Sooooooooooooo:
x+135=0.80x+144 subtract 0.80x and also 135 from both sides
x-0.80x+135-135=0.80x-0.80x+144-135 collect like terms
0.20x=9
x=45 oz-----------------amt of pure silver needed
CK
45+135=144+0.80*45
180=180

Hope this helps---ptaylor