TOPIC: Project Network Node (math question)

Hello,
I have a math question in regard of the Project network node. in the question the following information is given :
ES:21 LS: 37 LF:39
And the question is to calculate EF, Duration and Float.
I started the problem with calculating the Duration with the following formula LS-LF +1 (37-39 +1 ) and the answer is 3 days.
I got the answer wrong because I was following the Math rule and calculated -1 and I should have been doing 39-37 +1 = 3. But by switching the 37 and 39, I'm not following the math formula anymore... and this is what i don't grasp.

Is anyone able to provide some clarity and explain why we are aloud switch number .

Thank you,

What helps me is thinking of it practically, day 39, day 38 and day 37. So for duration it is 3 days.

The float is 16 and the Duration is 3
LS-ES = Float
37-21 = 16
LF-EF=Float
LF-EF=16
39-EF=16
EF=23
EF=ES+D-1
23=21+D-1
D=3

LF - LS = EF - ES -> 39 - 37 = EF - 21 => EF = 23
Duration = LF - LS + 1 = EF - ES + 1 = 3 days
Float = LF - EF = LS - EF = 39 - 23 = 37 - 21 = 16 days