Inside the seatpost to minimize impact on the movement of the bike while pedalling, an easy to swap in an unballasted seatpost later rather than faffing about removing glue.
Ignoring your second point, which is irrelevant to "physics problem" type questions, and allowing for your first by assuming an ISP to bring your answer in line with the rules, what physical parameter does your answer affect, and how does that effect the effect you claim?
I'm trying to give the additional weight minimal lateral movement throughout the pedal stroke, since the light bike moves under the overwhelming mass of the rider.
Inside the seatpost to minimize impact on the movement of the bike while pedalling, an easy to swap in an unballasted seatpost later rather than faffing about removing glue.