I saw this on the wall of my Math instructor friend. I took my Integral Calculus 25 years ago in UP Diliman as and Electrical Engineering student. At that time, I was really struggling with this subject.
However, surprise surprise!! I can still understand the computations and I actually did everything mentally!
To think I haven't used these concepts in years! Just goes to show how sharp our minds can still be - or maybe because now, without any worries about grades, I can just do these calculations for fun!