They have done it the way I would do it.
Note that if you have 3 number 1,5,7 the middle is the second, the middle of the first and third one (1+3)/2=2
There are 30 numbers so 3/4(1+30)=23.25
34 is the 23rd one but I want the 23.25th one so go 1/4 of the way from 34 to 36 which is 1/2 then add it to 34.