What is the constant speed of freddy and sam?

the constant speed, or "average rate of change" is their slope.

so  hmmm looking at Fred's graph, let's pick two points on the line, say 0,0 the origin and 8,2

[tex]\bf \begin{array}{lllll} &x_1&y_1&x_2&y_2\\ % (a,b) &({{ 0}}\quad ,&{{ 0}})\quad % (c,d) &({{8}}\quad ,&{{ 2}}) \end{array} \\\\\\ % slope = m slope = {{ m}}= \cfrac{rise}{run} \implies \cfrac{{{ y_2}}-{{ y_1}}}{{{ x_2}}-{{ x_1}}}\implies \cfrac{2-0}{8-0}\implies \cfrac{2}{8}\implies \cfrac{1}{4}[/tex]

now, for Sam, let's use the last two points in the table then,

[tex]\bf \begin{array}{lllll} &x_1&y_1&x_2&y_2\\ % (a,b) &({{ 20}}\quad ,&{{ 2\frac{1}{2}}})\quad % (c,d) &({{32}}\quad ,&{{ 4}}) \end{array} \\\\\\ % slope = m slope = {{ m}}= \cfrac{rise}{run} \implies \cfrac{{{ y_2}}-{{ y_1}}}{{{ x_2}}-{{ x_1}}}\implies \cfrac{4-2\frac{1}{2}}{32-20}\implies \cfrac{4-\frac{5}{2}}{32-20} \\\\\\ \cfrac{\quad \frac{8-5}{2}\quad }{12}\implies \cfrac{\quad \frac{3}{2}\quad }{\frac{12}{1}}\implies \cfrac{3}{2}\cdot \cfrac{1}{12}\implies \cfrac{1}{8}[/tex]