Billy buys light bulbs in packs of 8 for $20. The shipping cost is $10 regardless of the number of packs bought. Billy has only $120 to spend.
welp > _
First, take away the $10 from the $120 he has to spend, therefore he then has $110. Then, divide $110 by $20 and you will get 5.5. He can't get half of a pack so he can only get 5 packs. I hope this helped!
