6.2. Write the positive integers $1,2, \cdots, 100$ in increasing order on a blackboard in a row. Each time, it is allowed to erase several consecutive numbers and write down the number of erased numbers in their place (for example, erasing $13,14,15$, and writing 3). Can it be done after several such operations, so that the numbers left on the blackboard are 50,51?
6.2. Not possible.

The key lies in 50, 51 not being added numbers. Since it is impossible to erase 101 numbers, 51 must have been on the board initially, and all numbers to its right should have been erased, which is impossible.