Question 251732
Let x=amount of soil containing 45% sand that needs to be added
Now we know that the amount of pure sand in the soil that's added (0.45x) plus the amount of pure sand in the 1 cu ft(0.15*1) has to equal the amount of pure sand in the final mixture(0.35(1+x)).  So our equation to solve is:
0.45x+0.15*1=0.35(1+x)  get rid of parens
0.45x+0.15=0.35+0.35x subtract 0.35x and also 0.15 from each side of the equation

0.45x-0.35x+0.15-0.15=0.35x-0.35x+0.35-0.15  collect like terms
0.10x=0.20
x=2 cu ft-------------amount of soil containing 45% sand that needs to be added

CK
0.45*2+0.15*1=0.35(2+1)
0.90+0.15=1.05
1.05=1.05

Hope this helps---ptaylor