Stetson Inc.$\text{'}$s sales for the $2021$ were $$16$ million. Sales were $$8$ million $5$ years earlier.

$($a$)$ At what rate did the sales grow?

$($b$)$ Supose someone calculated sales growth for Stetson in part $($a$)$ as follows: "Sales doubled in $5$ years. This represents a growth of $100\%$ in $5$ years; dividing $100\%$ by $5$ results in an estimated growth rate of $20\%$ per year" Explain what is wrong with this calculation.