Question 286053
Let x=amount of water needed
Now we know that the amount of pure salt in the 20% solution(0.20*10) plus the amount of pure salt in the water that was added(0) has to equal the amount of pure salt in the final mixture(0.05(10+x)), so our equation to solve is:

0.20*10=0.05(10+x)  get rid of parens and simplify
2=0.5+0.05x  subtract 0.5 from each side
2-0.5=0.5-0.5+0.05x
0.05x=1.5   divide each side by 0.05
x=30 cm^3--------------amount of water needed

CK
0.20*10=0.05*40
2=2

Hope this helps---ptaylor