13. In a line, there are 20 numbers. It is known that the sum of any three consecutive numbers is greater than zero. Can we assert that the sum of all 20 numbers is greater than zero?
13. No. For example:

| -5 | -5 | 11 | -5 | -5 | 11 | -5 | -5 | 11 | -5 | -5 | 11 | -5 | -5 | 11 | -5 | -5 | 11 | -5 | -5 |
| :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- | :--- |