its ok
there are some tricks that can be helpful but there are also example that make it to complicated and hard. for example 45+54=99
the app says to add 4+5=9 then 9*11=99 why the hell would I do that when I can clearly see lining up the numbers in my mind they will both be 9s. it get even crazier with 3 digit numbers, taking a simple 3digit addition number and mixing multiplication and more addition, it just seams stupid to make it harder.
Finees about
Mencal - Mental Math Tricks