Balancing a chemical equation, but with an incorrect mole ratio
The balanced reaction is $$\ce{2AgNO3 +Cu -&gt; 2Ag +Cu(NO3)2}$$

Firstly, it's not necessary that all the reactants are depleted in the reaction.

There is the concept of a "limiting reagent" -- the reagent that gets depleted first and drives the extent of the reaction.

In this case, the limiting reagent is $\ce{AgNO3}$, and you will have half a unit of $\ce{Cu}$ left over.

In the general case, the limiting reagent need not be the one in the least quantity. For example, for $\ce{2A +B -&gt;C +D}$ , if $\ce{A:B}$ is 3:2, then $\ce{A}$ is still the limiting reagent as it will get depleted first.