Question 390126
Let x=amount of peanuts 
y=amount of macadamia nuts
z=amount of cashews 
x+y+z=20----------------------------------eq1
0.30x+0.90y+1.30z=20*0.68=13.6---------------eq2
y+z=x--------------------------------------eq3
substitute y+z=x into eq1 and we get
2x=20
x=10 oz-------------------------amount of peanuts needed
substitute x=10 into eq2 and eq3 and we get:
0.90y+1.30z=10.6----eq2a
   y+z=10-----eq3a
multiply eq3a by 0.90 and then subtract eq 3a from eq2a and we get:
0.40z=1.6
z=4 oz---------------------amount of cashews needed
substitute z=4 into eq 3a and we get:
y=6 oz--------------------------amount of macadamias needed

CK
0.30*10+0.90*6+1.30*4=13.6

3+5.4+5.2=13.6
13.6=13.6

Hope this helps---ptaylor