Try this one :
protected void txtqty_TextChanged(object sender, EventArgs e)
if (txtrate.Text != "")
txttamt.Text = (((Convert.ToInt32(txtrate.Text.Trim())) * (Convert.ToInt32(txtqty.Text.Trim())))).tostring();
What i did here is that i added a bracket at the beginning and another one at the end then converted the whole statement to string.So your error is that you are placing integers in txttamt which accepts strings.