Hello there!
To solve this, you want to find the unit rate (or cost per muffin) and multiply that by 9 to get the cost for 9 muffins.
$6.40 ÷ 4 muffins = $1.60 a muffin
So:
$1.60 a muffin x a total of 9 muffins = $14.40 for nine muffins.
I hope this was helpful! :)
