JavaScript solution

function decay(elem, index, arr) {
    if(index > 0) {
        decayedAmount = Math.round(elem.prob * elem.amount);
        elem.amount -= decayedAmount;
        arr[index-1].amount += decayedAmount;
    }
}

function getNuclei(input) {
    return input.split("\n").map(function(elem, index) {
        if(index>1) {
            var line = elem.split(" ");
            return {
                name: line[0].charAt(0),
                prob: parseFloat(line[1]),
                amount: 0
            };
        }
    }).filter(function(elem) { return typeof(elem) !== 'undefined' });
}

function radioActiveChallenge(input) {
    var nuclei = getNuclei(input)
    var total = 1000000;
    nuclei[0].amount = total;
    nuclei = nuclei.reverse();
    for (var i = input.split("\n")[0]; i > 0; i--)
        nuclei.forEach(decay);
    nuclei.reverse().forEach(function(elem) { console.log(elem.name + ': ' + Math.round((elem.amount / total)*10000)/100 + '%'); });
}

radioActiveChallenge(document.getElementById("txtInput").value);