Math(s)
 

♫ Teach your children well,
Their father's hell did slowly go by,
♫ And feed them on your dreams
The one they picks, the one you'll know by.

Back in the dark ages before everyone had a calculator in their pocket, I often had to do quick calculations in my head.
Percentages of 5% or multiples of 10% were OK, but odd ones a little tougher for a non-math(s) person.
I wish someone had pointed this out to me...

Very clever!

Not new math, just new mnemonic.

Caution...