4. Zero-sum test. A certain test consists of 26 questions. For each incorrect answer, five points are deducted from the test-taker, and for each correct answer, eight points are awarded.

The test-taker answered all the questions. How many questions did he answer correctly if the total number of points he received was zero?
4. The ratio of the number of answers of each type is equal to the inverse ratio of the corresponding points. Therefore, the number of correct answers is $\frac{5}{5+8} \cdot 26=10$.

[M. M., 31, 237 (March 1958).]