Greg has 60 building blocks. his sister gives him 17 more blocks. he uses 38 blocks to make a tower. how many blacks are not used in the tower?

Greg has 39 remaining blocks that are not used in the tower.

Step-by-step explanation:

According to the question, Greg started with 60 building blocks.  However, his sister gives him 17 more blocks.  The key word 'more' means we need to add these blocks to the original 60:  60 + 17 = 77.  Greg then decides to use 38 blocks to make a tower.  Since he is using these blocks from his total of 77, we need to subtract this number to see how many he has left:  77 - 38 = 39.  This means that Greg has 39 blocks left that were not used to build his tower.