Math w/o calculator.
For very fast division/multiplication you need to find log w/o thinking. Aka -- memorize all log values up to two digits.
It's far easier than you think.
There are 90 log values that you need to learn : 10 to 99.
Of these, for the last 30 numbers, 70 to end of list you need to add half the distance to 100 with that number for its log value.
Ex. log 70 = distance to 100 is 30 half of which is 15.
Therefore log 70 is 85.
And for the mid 30 numbers, starting from 30 ending to 60, you blindly add 20 with the number and get done.
Like log 55 = 45+20 = 65.
These two simple rules cover 60 from a list of total 90 to learn.