7. Letters (from 6th grade. 1 point). At the post office, a courier sends 100 letters. Letters weighing more than 100 g are not accepted. The cost of sending depends on the weight of the letter. The minimum cost (for letters weighing up to 20 g inclusive) is 22 rubles, and for each subsequent full or partial 20 g, an additional 2 rubles 50 kopecks is added (see table).

The first postal clerk ${ }^{1}$ believes that it is necessary to weigh each of the 100 letters separately, find the cost of sending each according to the table, and then add up all the amounts.

| Postal tariffs for simple letters |  |
| :---: | :---: |
| Weight, r | Price, $\mathrm{p}$ |
| up to 20 | 22.00 |
| from 21 to 40 | 24.50 |
| from 41 to 60 | 27.00 |
| from 61 to 80 | 29.50 |
| from 81 to 100 | 32.00 |

The second clerk suggests weighing all the letters at once, finding the average weight of a letter, determining the cost of sending according to the table, and multiplying the result by 100. She says this will give the same total, only faster. Is the second clerk right?
Solution. Not correct. Suppose that two letters weigh 100 g, and all the others weigh 20 g. Then the first operator will calculate that the courier should pay $98 \cdot 22 + 2 \cdot 32 = 2220 \text{ p}$.

The second operator will find that the average weight of a letter is

$$
\frac{20 \cdot 98 + 100 \cdot 2}{100} = 21.6 \text{ g, }
$$

and its delivery costs 24.5 p. Therefore, she will charge $24.5 \cdot 100 = 2450$ rubles.

There are other examples showing that different calculation methods can yield different results.

Answer: no, not correct.