7. Petya sequentially writes down integers, starting from 21, such that each subsequent number is 4 less than the previous one, and Vasya, looking at the current number, calculates the sum of all the numbers written down by this point. Which of the sums found by Vasya will be closest to $55?$

Answer: 56.
Solution. Let's create a table, in the first row of which we will record the numbers following Petya, and in the second row we will place the results of Vasya's calculations.

| 21 | 17 | 13 | 9 | 5 | 1 | -3 | -7 | -11 | $\cdots$ |
| :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :---: | :--- |
| 21 | 38 | 51 | 60 | 65 | 66 | 63 | 56 | 45 | $\cdots$ |

Since the numbers that Vasya will write down further are negative, the sum calculated by Kolya will decrease, moving away from 55. Therefore, the closest to 55 will be the sum equal to 56.