Ivan and Adeline are in a classroom with a chalkboard. They are standing on different halves of the board, and on each half, the number $2$ is written. When Ivan's teacher gives a signal, Ivan multiplies the number on his side of the board by $-2$ and writes the answer on the board, erasing the number he started with. Adeline does the same on each signal, except that she multiplies by $2$. The teacher gives 10 signals in total. How many times (including the initial number) do Ivan and Adeline have the same number written on the board (including at the beginning)?

The number of times Ivan and Adeline have the same number written on the board is 6.

Step-by-step explanation:

Consider the procedure as follows:

Consider the numbers on each half of the board:

          Ivan                            Adeline

             2                                     2

      2 × -2 = -4                        2 × 2 = 4

     -4 × -2 = 8                         4 × 2 = 8

      8 × -2 = -16                      8 × 2 = 16

   -16 × -2 = 32                      16 × 2 = 32

   32 × -2 = -64                    32 × 2 = 64

  -64 × -2 = 128                   64 × 2 = 128

  128 × -2 = -256               128 × 2 = 256

-256 × -2 = 512                256 × 2 = 512

  512 × -2 = -1024              512 × 2 = 1024

-1024 × -2 = 2048           1024 × 2 = 2048

Thus, the number of times Ivan and Adeline have the same number written on the board is 6.

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