Set the default selectedindex = 0.
Here
ddlAdult is Adult dropdown id.
ddlChild is Child dropdown id.
txtTotalPrice is textbox id
If(ddlAdult.SelectedIndex != 0 && ddlChild.SelectedIndex != 0)
{
txtTotalPrice = (Convert.ToInt32(ddlAdult.SelectedValue) * adultPrice) + (Convert.ToInt32(ddlChild.SelectedValue) * childPrice);
}