# Back of the envelope calculations help.

1. Feb 1, 2012

### nascentmind

Hi,

How do I develop and train to do quick back of the envelope calculations? How have you developed these techniques? I am not quick in it and make silly mistakes.

2. Feb 1, 2012

### jim hardy

force yourself to rigorously write down each step, as you did in junior high school math.

perhaps i have advantage - grew up in days of slide rule where first thiing you had to reduce a problem to something manageable. And you had to keep track of decimal point yourself.
An early complaint when calculators came around was ''The kids dont think anymore. If they divide twelve by sixty and get twenty for an answer, they dont question it."

So it's a question of developing good habits.
Always do reasonability checks.

Haste makes waste.

3. Feb 2, 2012

### nascentmind

That's the exact problem I have i.e. keeping track of decimal points and also the 0's.

4. Feb 2, 2012

### jim hardy

""That's the exact problem I have i.e. keeping track of decimal points and also the 0's.""

we are rushed today in everything we do. i think it's the computers...

form the habit of writing all your numbers in scientific notation,
so when you multiply or divide you're dealing with numbers arout 0.1 to 10 .

then collect the exponents.

always do them on paper (that's what the envelope is for)
and NEATLY paying attention to alignment.. (Do they teach mechanical drafting in jr high anymore?)

And do them twice for cross-check. That avoids a lot of embarassment.

Henry Thoreau said: "The most valuable thing a man can possess
is a determination not to be hurried."

old jim