Greg bought 2 boxes of balloons. He used half of them to decorate his yard. He used 40 to decorate his porch. He used the rest inside his house. How many balloons did he use inside?

You need other information like how many balloons per box, we suppose it is 100. Then you know that Greg use one box to decorate his yard. Then you can use 100 minus 40 to get the answer 60.