Suppose Samantha dies this year with a gross estate of $15 million and no adjusted prior gifts. Calculate the amount of estate tax due (if any) under the following scenarios:

1. Samantha's husband, Mark, is the sole beneficiary.
2. Mark receives $10 million from the estate and the remainder is donated to the American Red Cross.
3. Mark receives $10 million from the estate and their son, Luke, receives $5 million.
4. Mark receives $10 million and $5 million is placed in a trust. The trustee is required to pay income to Mark for her life and the remainder to their son, Luke.