Question 1101892
An industrialist has 450 litres of a chemical which is 70% pure.  He mixes it with a chemical of the same type but 90% pure so as to obtain a mixture which is 75% . pure. Find the amount of the 90%  pure chemical used. 
 
The industrialist has no pure chemical as far as we know.
He has on hand a mixture which is 70% and a mix that is 90% and wants to blend them to get a mix that is 75%
Pure means 100%
The 70% mix is a blend of 30%*450 = 125 L something else and 70 %*450 =315 L pure 
Otherwise, the math presented is correct.

We have on hand 450 L of 70% solution.
Using the method of alligation.


<pre>
90         5 5/15*450=150 L of 90% solution
      75            600 L of 75% solution
70         15        450 L of 70% solution
           20
</pre>

check
0.9 * 150 + 0.7 * 450 = 0.75*(450+150)
135.0 + 315.0 = 0.75*(600)
450.0 = 450.0
ok