Henry has 13 post-it notes more than Tiffany. How many post-it notes must Tiffany get from Henry so that she has 21 post-it notes more than him?

	Henry	Tiffany
Comparing Henry and Tiffany at first	13 more
Before
Change	- ?	+ ?
After
Comparing Henry and Tiffany in the end		21 more

If Henry gives 13 post-it notes to Tiffany, she will have 13 more post-it notes than him. However, Tiffany must have 21 more post-it notes than Henry in the end. So, Henry has to give Tiffany more post-it notes.

Number of more post-it notes that Henry has to give to Tiffany
= (21 - 13) ÷ 2
= 8 ÷ 2
= 4

So after giving 13 post-it notes to Tiffany, Henry has to give her another 4 more post-it notes so that she has 21 post-it notes more than him.

Total number of post-it notes that Tiffany needs to get from Henry
= 13 + 4
= 17

Answer(s): 17