Hi Hannah,
Lets find the cost per ounce
there are 16 ounces in a pound
so
5 lbs = 5(16)ounces = 80 ounces
so the cost per ounce is
2.90/80
= .03625 per ounce
and then 3 ounces equal 3 * .03625
= .10875 for three ounces
Hope this helps