Question 155010
I THINK YOU NEED TO RECHECK YOUR PROBLEM.  I THINK THE "40%" SHOULD BE "60%".  ANYWAY, I WILL MAKE THAT ASSUMPTION.

Let x=amount of 60% solution needed
Then 40-x=amount of water needed

Now we know that the amount of pure boric acid in the water(0) plus the amount of pure boric acid in the 60% solution (0.60x) has to equal the amount of pure boric acid in the final mixture(0.10*40).  So our equatuion to solve is:

0.60x=0.10*40 
0.60x=4  divide each side by 0.60
x=6.67 liters  --------------of the 60% solution
40-x=40-6.67=33.33 liters-------------of water

Ck
6.67*0.60=40*0.10
4=4


Hope this helps---ptaylor