13. an abstract algebra teacher intended to give a typist a list of nine in- tegers that form a group under multiplication modulo 91. instead, one of the nine i

13. An abstract algebra teacher intended to give a typist a list of nine in- tegers that form a group under multiplication modulo 91. Instead, one of the nine integers was inadvertently left out, so that the list ap- peared as 1, 9, 16, 22, 53, 74, 79, 81. Which integer was left out? (This really happened!)