The answer would be 100 because 100x30 equals to 3,000
I hope this is the answer you were looking for
4cm•2=8cm which is the length of the red paper so therefore Red paper= 4cmx8cm.
Then your going to take 3cm-4cm= 1cm
Next your going to solve for 7cm-8cm=1cm
Which leaves you with an answer of 1cmx1cm more of visible red paper.
I think you mean,

You can use the usual formula,

That in this case is,

Where I have used,