Question

How do you define a variable and write an algebraic expression for the phrase: 3 increased by a number?

Answer 1

Depends on context:

Explanation:

Let the variable be ...... ( what ever you chose)

This variable could have definable limits or belong to a particular set of number types

You could say "There exists a number n such that .....

or you could write:`" "{n, n in R}`

On the simpler context

Let the variable be n such that `n>0`
and that `3+n`......... and whatever follows. Perhaps "increases".

n would have to be positive otherwise it would not "increase" 3 when added.