Grandpa was in his crazy math mood again.
"using rot13 and your math knowledge prove to me that
505 = 1"
Really? Can you?
Here's one possible solution:
1. $505 = 1$ : Given
2. $DV = 1$ : By Romanizing left
3. $V = 1/D$
4. $CV = C/D$ : Multiply through by 100
5. $(PI)= C/D$ : By Rot(13)ing left
6. $Dπ = C$
By the circumference formula for a circle (C= πD),the left and right are equivalent
In Roman numerals 505 is DV. If we use rot13 on these two characters, we get QI.
Qi is the circulating life force whose existence and properties are the basis of much Chinese philosophy and medicine. It allows us to say that we are one with the universe. In this way, 505=1
The rot13 translation of "one" is "bar". The only connection between "bar" and 505 that I could find is this very obscure definition given in urban dictionary (which was entry #4 and has more downvotes than upvotes), which says that it's slang for getting a drink at a bar with friends after work. Perhaps it's a really old outdated slang term, which is why your grandpa used it.
Working out possible solutions using rot13 and trig:
I feel gross and like I'm doing something wrong doing this but:
505 = 5 * 101 => V * CI
rot13(V * CI) = I * PI => i * pi (getting pi honestly is the only reason I think this might be the right direction.
Here's where the rot13(gevt) comes in I guess? Not sure where to proceed from there.