$3.87÷3 is $1.29, so you are spending $1.29 per pound on that deal.
$6.65÷5 is $1.33, so you are spending $1.33 per pound on that deal.
So, 3 pounds for $3.87 is a better buy because you are spending $1.29/pound instead of $1.33/pound.
Hope this helps! -Chat