Take a good hard look at these diagrams.
Study these relationships very carefully and you will nail it!!!
[attachment=1:3kfqk80k]the expsoure triangle 2.jpg[/attachment:3kfqk80k]
photo by unleashingmephotography
[attachment=0:3kfqk80k]advanced exposure triangle 2.jpg[/attachment:3kfqk80k]
photo by bangdoll