Answer
A box that makes $34$ servings.
Work Step by Step
A box of $20$ servings per $2.15$ dollars
$\frac{20}{2.15} = 9.3$ servings per dollar
A box of $34$ servings per $3.35- 0.5 = 2.85$ dollars
$\frac{34}{2.85} = 11.9$ servings per dollar
A box of $65$ servings per $10.41- 0.5 = 9.91$ dollars
$\frac{65}{9.91} = 6.6$ servings per dollar
Your best buy is a box that makes $34$ servings.
