Answer:
Greg had
balloons to decorate inside the house.
Explanation:
Lets say Greg has N number of balloons per box.
So, in total he had 2N balloons.
He used half of 2N to decorate the yard.
So, we have N balloons left.
Out of N, he used 40 to decorate his porch.
Then again the left number of balloons will be =

Therefore, Greg had
balloons to decorate inside the house.