20 moles of A and 14 moles of B are mixed and allowed to react according to the equation, $$ A + 2 B \rightarrow 3 C $$ What is the maximum number of moles of C which could be prepared?

To identify the limiting reagent, compare moles divided by stoichiometric coefficients: for A, $20/1=20$; for B, $14/2=7$. Since 7 is smaller, B is the limiting reagent. From $2B \to 3C$, 14 moles of B produce $\dfrac{3}{2}\times14=21$ moles of C.