20.1. A boy wrote a certain number. In the second place, he wrote the number 7, in the third place - the difference between the second and the first number, in the fourth place - the difference between the third and the second number, and so on. What number will be in the $200-\mathrm{m}$ place?

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20.1. Let the first number be denoted by $x$. Then, following the condition of the problem, we get the following sequence of numbers: $x, 7, 7-x, -x, -7, x-7, x, 7, \ldots$ 18

As we can see, the seventh and eighth numbers repeat the first and second, i.e., we have a periodic repetition of numbers with a period of 6. Since 200 divided by 6 leaves a remainder of 2, we will have the same number on the two hundredth place as on the second place, i.e., 7.