Man this takes me back...and I was **** at Chemistry...sigh.
As said above your answer in dm and theirs in cm is the same, albeit I think you see cm(3) more often, doesn't matter though.
Last one you haven't added 10% in your calculation, you seem to have added 0.1 to 3.0875 to get your 3.1875. Where as 10% of 3.0875 is 0.30875, so 3.0875+0.30875=3.396, or just multiply 3.0875 by 1.1 as they do.
Btw out of curiosity what exam is this from?