Coprimes

Two numbers are coprime if the only factor common to them is 1 or -1.

$26$ $and$ $56$

$26$ $:$ $1,2,13,26$

$56$ $:$ $1,2,4,7,8,14,28,56$

$21$ $and$ $22$

$21$ $:$ $1,3,7,21$

$22$ $:$ $1,2,11,22$

$24$ $and$ $28$

$24$ $:$ $1,2,3,4,6,8,12,24$

$28$ $:$ $1,2,4,7,14,28$

It shows that only 21 and 22 are coprimes because the only factors common to them is 1.