Answer :
[tex]55|5\\11|\fbox{11}\\.\ 1|\\----------\\77|7\\11|\fbox{11}\\.\ 1|\\-----------\\\\\boxed{GCF(55;\ 77)=11}[/tex]
The factors of 55 are 1, 5, 11, and 55 .
The factors of 77 are 1, 7, 11, and 77 .
The common factors of 55 and 77 are 1 and 11 .
The greatest one is 11 .
The factors of 77 are 1, 7, 11, and 77 .
The common factors of 55 and 77 are 1 and 11 .
The greatest one is 11 .
