Question: Calculate Q1 (lower quartile)

There are 50 data entries so im not going to type them all.

Answer:

n = 50, so (n+3)/4=13.25

Therefore Q1 is a quarter of the distance between the 13th an 14th sorted data values.

I dont understand why we add three to our number of data values. (Worried) Can sombody please explain this. PLease please please!