Concept:
Each coin supports the weight of all coins placed above it. Force on a coin is due to the weight of coins above it.
Step 1: Identify position of the coin.
The $6^{\text{th}}$ coin from the bottom has:
\[
10 - 6 = 4 \text{ coins above it}
\]
Step 2: Calculate total force on the coin.
Each coin has weight $mg$, so total force due to coins above:
\[
4 \times mg = 4mg
\]
Step 3: Conclusion.
Force on the $6^{\text{th}}$ coin is:
\[
4mg
\]