Thanks will look again. I had assumed that if only one section of 20 cells is to be repositioned the other 149 cells stay where they are.
I also assumed that the 20 repositioned cells must completely separate the other two parts or why would you say cut into three pieces. If they contained adjacent cells and did not move then they effectively remain as one piece.
Then my problem. If all cells are visible after repositioning then section cut out and repositioned must be symmetrical and I can not see how to do this if 13 out of 20 are involved in the transection leaving an odd numbered remainder.
Clearly missing something. Will have another look tomorrow.