9.1. How much of a $5 \%$ and a $20 \%$ salt solution in water should be taken to obtain 90 kg of a $7 \%$ solution
Answer: 78 kg of 5% solution and 12 kg of 20% solution.

Solution. Let the mass of the 5% solution be $x$ kg, the mass of the 20% solution will be $90-x$ kg, and the total mass of salt in the 5% and 20% solutions is equal to the mass of salt in 90 kg of 7% solution: $\frac{5}{100} x+\frac{20}{100}(90-x)=\frac{7}{100} 90$, from which $1800-15 x=630$ and $x=78$.

Grading criteria. Answer found by guessing with verification: 1 point. Equations written but not solved: 2 points.