function a_times_b(form) {
a=eval(form.a.value)
b=eval(form.b.value)
if (isNaN(b) == true) {b = "0"; }
if (b > "20") {c = ".30"; }
else if (b > "10") { c = ".30"; }
else if (b > "0") { c = ".30"; }
else { c = "0"; }
d=a*b*c
form.ans.value= "$" + d.toFixed(2)
}