[рони]

Skypidar,
obj заполнить для всех индексов spisok15

function changeText() {
    var t, tt, s = +document.getElementById("s1").value||0,
        indx = document.getElementById("spisok15").selectedIndex,
        obj = {
            0: {
                v: 20,
                m: 3,
                kr: .5
            },
            1: {
                v: 10,
                m: 2,
                kr: 1
            }
        }[indx]; alert(indx);
    t = s * obj.m;
    tt = s / obj.v;
    document.getElementById("otv").innerHTML = t;
    document.getElementById("otv1").innerHTML = tt
};