seems like an overcomplicated and impractical (for quick calculations) form of the formula tbh,

Let f(n) be the level

f(n) = 20+30+...+10(n+1)
=10(2+3+4+...+n+1)
=10 (1+2+3+...+n+1) -10
= 10(n+1)(n+2)/2 - 10
=5(n+1)(n+2) - 10