CLUE WORDS FOR ADDITION:
altogether
total number
sum
in all
perimeter
combined
how many
spend/spent
joined
increase
inflate

CLUE WORDS FOR SUBTRACTION:
left
difference
remained
how much more
how many more...than
how much change
less
change
"er" words
decrease
deflate

CLUE WORDS FOR MULTIPLICATION:
twice
product
each-before question
area
times
per-before question
every-before question

CLUE WORDS FOR DIVISION:
separate
half
equal number
divided something into groups
quotient
share something equally
per-in question
a piece-in question

"Each" means divide only if you are never going to have more than the stated number.
*For example: Doug had 12 goldfish. He placed an equal number of fish in each of his 4 bowls. How many fish are in each bowl? (You are never going to have more than 12 fish or more than 4 bowls - so you divide: 12 divided by 4 = 3 fish in each bowl.)

ROUNDING:
about
estimate