You beat by a few minutes. I wrote a Chromium extension for it:

cs.js:

    (function(){var b;var a=function(){if(b!==undefined){var e=document.querySelector("input#answer");
    e.value=b;e.dispatchEvent(new Event("keyup"))}};var c=function(e){var h=e.length;
    var f;while(h--){f=e[h];if(!f.addedNodes||!f.addedNodes.length)
    {continue}if(f.addedNodes[0].nodeType!==3){continue
    }var j=f.addedNodes[0].textContent.match(/^\s*(\d+)\s*(\S)\s*(\d+)/);if(!j){continue
    }var k=parseInt(j[1],10);var l=j[2];var g=parseInt(j[3],10);switch(l){case"+":b=k+g;
    break;case"-":b=k-g;break;case"x":b=k*g;break;case"÷":b=k/g;break}setTimeout(a,500);
    return}};var d=function(){var e=new
     MutationObserver(c);e.observe(document.querySelector("div#question"),
    {attributes:false,childList:true,characterData:false,subtree:false})
    };window.addEventListener("load",d)})();

manifest.json

    {
    "manifest_version": 2,
    "name": "Speedsums Helper",
    "version": "0.1",
    "description": "Help be good at SpeedSums",
    "author": "wreegab",
    "content_scripts": [{
        "matches": ["http://www.speedsums.com/*"],
        "js": ["cs.js"],
        "run_at": "document_start"
    }]
    }

Caveats: Need to answer the first question. Need a time out to prevent exhausting the list of questions (it stops at 119)

It's Hacker News after all...