Question

How much `40%` solution and `70%` solution does a scientist need to get `30` liters of `60%` solution?

Answer 1

Scientist needs `10` liters of `40%` solution and `20` liters of `70%` solution to get `30` liters of `60%` solution.

Explanation:

Whenever you have such problems where two mixes with different strengths are to be mixed to get a third mix whose strength is something between the strength of two mixes,

1. First write the two strengths with space between them (as shown inn figure below.
2. Then in between write the desired strength.
3. Write the differences as shown by crossed lines, and
4. simplify the ratio.
   
   This is the desired ratio. Here it `1:2` and as we need `30` liters of solution, we need `10` liters of `40%` solution and `20` liters of `70%` solution.