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The mint mark for the 1909 Half Eagle is located on the reverse {eagle} side of the coin just to the left of the arrow points.
The value of a 20 mark gold coin from 1909 would depend on its condition, rarity, and current market demand. You would need to consult with a numismatic expert or appraiser to get an accurate valuation for this specific coin.
Unless it has an O mint mark, about $300 for its gold content. If it's a 1909-O, it's worth about 2.5X as much as a collectible coin.
The mint mark on a $5 gold coin from that era is on the reverse (tails) side, just below the eagle.
The P is the mint mark for Philadelphia.
On the reverse just to the left of the arrows
depending on condition, about $300
The US has never used an "E" mint mark. You may be looking at a "D" (Denver) or "S" (San Francisco). There's more information at the Related Question.
Please post a new question with the coin's date and mint mark.
Assuming the coin is circulated and no mint mark retail values are $312.00-$420.00
From about $1300 to $75,000 depending upon the mint mark and the condition of the coin.
Starting in 2009 the date & mintmark was moved to the edge of the coin, it's not gold and it's only a dollar.