5. A store sold some pens on the first day. On the second day, after reducing the price of each pen by 1 yuan, it sold 100 more pens. On the third day, after increasing the price of each pen by 3 yuan compared to the previous day, it sold 200 fewer pens than the previous day. If the store made the same amount of money each day, what was the price of each pen on the first day? $\qquad$ yuan.
【考点】Application Problem
【难度】Hehehe
【答案】 4 yuan
【分析】Let the price of each pen on the first day be $x$ yuan, and the number of pens sold be $n$. We have $\left\{\begin{array}{l}n x=(x-1)(n+100) \\ n x=(x+1)(n-100)\end{array}\right.$, which leads to $\left\{\begin{array}{l}100 x=n+100 \\ 100 x=2 n-200\end{array}\right.$, solving these equations yields $\left\{\begin{array}{l}n=300 \\ x=4\end{array}\right.$