Step 1 : Find prime factorization of each number.

$$\begin{aligned}42 =& 2\cdot3\cdot7\\[8pt]56 =& 2\cdot2\cdot2\cdot7\\[8pt]46 =& 2\cdot23\\[8pt]68 =& 2\cdot2\cdot17\\[8pt]104 =& 2\cdot2\cdot2\cdot13\\[8pt]128 =& 2\cdot2\cdot2\cdot2\cdot2\cdot2\cdot2\\[8pt]\end{aligned}$$

(view steps on how to factor 42, 56, 46, 68, 104 and 128. )

Step 2 : Put a box around factors that are common for all numbers:

$$\begin{aligned}42 =& \color{blue}{\boxed{2}}\cdot3\cdot7\\[8pt]56 =& \color{blue}{\boxed{2}}\cdot2\cdot2\cdot7\\[8pt]46 =& \color{blue}{\boxed{2}}\cdot23\\[8pt]68 =& \color{blue}{\boxed{2}}\cdot2\cdot17\\[8pt]104 =& \color{blue}{\boxed{2}}\cdot2\cdot2\cdot13\\[8pt]128 =& \color{blue}{\boxed{2}}\cdot2\cdot2\cdot2\cdot2\cdot2\cdot2\\[8pt]\end{aligned}$$

Step 3 : Multiply the boxed numbers together:

$$ GCD = 2 $$

GCD Calculator