This looks like a parabola whose vertex is x=40. However, the curve is not exactly symmetric. so you can add some negative exponential function which will smooth out the curve...

Interesting thought! Shabbir Ahmed Thanks. I'll give it a try.

Thanks Behzad Rouhanizadeh

I came across the same senario, did you find the solution!

I have a multiple similar curves, and I would find the unique polynomial equation that will fit all those curves ! Tousif Ahmed Behzad Rouhanizadeh