Let the length of a domino be 1. On the edge of a table, a single domino can extend out a distance of 1/2 before being unbalanced. A stack of two dominos can extend out a distance of 3/4. How large of an overhang can you create with 5? 10? N? How many dominos are needed before the top domino is one full horizontal length away from the edge of the table, for a full overhang length of 2? Each domino must be placed in the same orientation (don't rotate the dominos or stand them on end.)
Extension: Does it make a difference how far the overhang reaches if you can place dominos on top as extra weight? For example, the same 2-domino distance of 3/4 can be reached when the bottom domino extends out and the top domino is placed back as ballast:
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Source: Past puzzle experience. This can also be done with playing cards.