Question 55016
Since each sheet when folded in half would create what looks like a card, it has for pages to it.  The front part of first piece would be pages 1 and 2 while the back part would be pages 79 and 80.


01 02 ..... 03 04 ....
80 79 ..... 78 77 ....
Each pair of pages adds to 81 ... 1+80 .. 2+79 ... 3+78 ... etc.


Keeping this rule in mind and the fact that page 11 would be the "front" of it's sheet (12 would be the other side, just like with pages 1 and 2) we can then find the number that adds to 11 to make 81


81 - 11 = 70
81 - 12 = 69


So the pages that are missing are 11, 12, 70 and 69