To solve this is you have to find the price per ounce for each item by simply dividing the price by the ounces.
For the oat cereal divide $3.90 by 15
3.90/15
.26 per ounce
For the rice cereal divide $3.30 by 11
3.30/11
.30 per ounce
The oat cereal costs less per ounce since it is only .26 per ounce whereas the rice cereal is .30 per ounce.
Hope this helps!