To determine the constant of proportionality, we need to look at the ratio of the cost to the number of baseballs.
First, let's determine the ratio for the given data points:
1. For 10 baseballs costing [tex]$5:
\[
\frac{\text{Cost}}{\text{Number of Baseballs}} = \frac{5}{10} = 0.5
\]
2. For 12 baseballs costing $[/tex]6:
[tex]\[
\frac{\text{Cost}}{\text{Number of Baseballs}} = \frac{6}{12} = 0.5
\][/tex]
3. For 22 baseballs costing $11:
[tex]\[
\frac{\text{Cost}}{\text{Number of Baseballs}} = \frac{11}{22} = 0.5
\][/tex]
In each case, the ratio of the cost to the number of baseballs is the same, which confirms that the relationship between the cost and the number of baseballs is proportional.
Therefore, the constant of proportionality is:
[tex]\[
\boxed{0.5}
\][/tex]
