Here is a $ 0.050 \; M $ solution of the weak base pyridine $(C_5H_5N$ , $K_b$ $=$ $1.7\cdot 10^{-4})$
$C_5H_5N$ $ +$ $H_2O$ 
$HC_5H_5N^+$ $+$ $OH^-$
What are the equilibrium molarities of each species ?
a) $x\;M$, $(0.050-x)\;M$, $x\;M$ b) $(0.050-x)\;M$, $x\;M$, $x\;M$ c) $x\;M$, $x\;M$, $x\;M$ d) $(0.050-x)\;M$, $(0.050+x)\;M$, $x\;M$
Answer: b) $(0.050-x)\;M$, $x\;M$, $x\;M$