I am trying to put together an expression that looks at several different possible values:

What i want:

If Speed > 70 then = Speed - 30

If Speed is lower than 70 but higher than 30 then = Speed -15

If Speed lower than 30 then = Speed

I put this as my expression which i know is wrong but i cannot figure it out:

IF([Speed] > 70, [Speed] - 30, [Speed]), AND IF([Speed] < 70, [Speed] - 15, [Speed])