how much is my morgan 1885 dollar worth?

im guessing 20$

The mint mark on a Morgan Dollar is on the reverse (Eagle) side, above the “D” and “O” in DOLLAR. You can see its location in this picture:

Here’s a retail pricing guide showing the prices that dealers might charge for 1885 (and other) Morgans in various grades (conditions), noting that the 1885 and 1885-O are common; the 1885-S has a slight premium in lower grades (conditions) and becomes highly valued in the highest grades; and the 1885-CC is highly valued in all grades by collectors:

Values vary by grade (condition). A guide to grading Morgan dollars:

At a minimum, this coin is worth its silver value. Coinflation.com provides a guide to the values of US and Canadian silver coins, including Morgan and Peace Silver Dollars, which is adjusted regularly to reflect changes in the spot silver price:

Since as-minted Morgan and Peace dollars contain .7736 Troy ounces of pure silver, their silver value is approximately $13, at a current silver price, as of this writing, of $17 per Troy ounce. (Well-circulated coins contain perhaps 1-5% less silver due to wear, but even a very worn Morgan should contain at least $12 in silver at current silver prices.)