In stories and poems, write numbers as words if they are below 101 or are fractions: otherwise write them as numbers.
Ford Prefect was desperate that any flying saucer at all would arrive soon because fifteen years was a long time to get stranded anywhere, particularly somewhere as mindboggingly dull as the Earth.
In math stories, you can use either numbers or words as long as you are consistent.
Use commas in a large number to break it into groups of three digits.
The Hitchhiker's Guide to the Galaxy says that if you hold a lungful of air you can survive in the total vacuum of space for about thirty seconds. However it goes on to say that what with space being the mind boggling size it is the chances of getting picked up by another ship within those thirty seconds are two to the power of 267,709 to one against.
There are several ways to write money: just pick one and stay consistent.
6 hundred dollars six hundred dollars $6 hundred $600
When cents are involved, NEVER use a dollar sign and cents sign in the same number.