Eight more than a number is 16.

I agree with Melody. Just rename "a number" "x", set up what it equals, and add (plus) or subtract (minus) the same value on both sides of the equality sign (=). 

Have a go! We'll help you if you get stuck. Or read my answer on "A number minus 3 is -8".

Come on Rochelle, you can at least guess at the answer.  

Then we can look at your guess if you want us to.

You would learn better that way too.