It takes 1/2 of box of nails to build bird house. if you wanted to build 5 bird houses, how many boxes would you need?

For every birdhouse, you need 1/2 of a box of nails to build it. 

Using that logic we can assume that for every birdhouse you add 1/2 to get to your final answer. 

1 birdhouse:     2 birdhouses:    3 birdhouses:    4 birdhouses:    5 birdhouses:

1/2                     2/2                     3/2                      4/2                     5/2

Now, we know that the answer is that we need to have 5/2 boxes of nails to build 5 birdhouses.

5/2 can also be converted into 2 1/2.

So 2 1/2 boxes of nails would be needed is the final answer