Answer
The package of 27 is the best buy.
Work Step by Step
To find the best buy compute the price per diaper for each package. Divide the package price by the number of diapers.
$\frac{11.71\ dollars}{1\ pkg}\times\frac{1pkg}{27 diaper}=\frac{11.71\ dollars}{27\ diaper}=\frac{11.71\ dollars\div27}{27\ diaper\div27}=\frac{0.43\ dollars}{1\ diaper}$
$\frac{19.98\ dollars}{1\ pkg}\times\frac{1pkg}{31 diaper}=\frac{19.98\ dollars}{31\ diaper}=\frac{19.98\ dollars\div31}{31\ diaper\div31}=\frac{0.64\ dollars}{1\ diaper}$
$\frac{29.99\ dollars}{1\ pkg}\times\frac{1pkg}{56 diaper}=\frac{29.99\ dollars}{56\ diaper}=\frac{29.99\ dollars\div56}{56\ diaper\div56}=\frac{0.54\ dollars}{1\ diaper}$
