To find the total width of 30 2-by-4 pieces of wood laid side-by-side, we first note that the actual width of a 2-by-4 is 3.5 inches.
Now, we can calculate the total width:
\[ \text{Total width} = \text{Number of pieces} \times \text{Width of each piece} \]
\[ \text{Total width} = 30 \times 3.5 , \text{inches} \]
Calculating this:
\[ 30 \times 3.5 = 105 , \text{inches} \]
Thus, the total width of 30 2-by-4s laid side-by-side is:
\[ \boxed{105} , \text{inches} \]