170. Antifreeze. The radiator of a car with a capacity of 21 quarts* is filled with an 18% alcohol solution. How many quarts of this solution need to be drained from the radiator so that, by replacing them with the same amount of 90% solution, a 42% alcohol solution is obtained in the end.
170. The percentage of alcohol in the old solution differs from the percentage of alcohol in the new (or mixed) solution by $-24 \%$, and the percentage of alcohol in the solution added to the radiator differs from the percentage of alcohol in the new solution by $+48 \%$. Therefore, for each quart of $90 \%$ solution added to the radiator, there should be 2 quarts of the old $18 \%$ solution. Hence, $\frac{1}{3}$ of the total volume, or 7 quarts of the old solution, should be drained from the radiator.

[B. E. Mitchell, M. M., 26, 153 (January 1953).]