বলা হচ্ছে যে, দুটি ঘড়ির একত্রে ক্রয়মূল্য 840 টাকা। একটি ঘড়ি 16% লাভে এবং অপরটি 12% ক্ষতিতে বিক্রয় করলে লাভ/ক্ষতি কোনটাই হয় না। প্রতিটি ঘড়ির ক্রয়মূল্য কত?

Let, the cost price of one watch is x Tk. 

The cost price of another watch is (840-x) Tk.

 Selling price at 16% profit=$x+16\% of x= x+\frac{16x}{100}=\frac{116x}{100}TK.$

Again, price at 12% loss

$$\begin{gathered} = (840-x)-12\% of (840-x) Tk \\ =\left(840-x\right)-\frac{12\left(840-x\right)}{100}TK. \\ =\left(840-x\right) - \frac{2520-3x}{25}TK. \\ = \frac{2100-25x-2520+3x}{25}TK. \\ = \frac{18480-22x}{25}TK. \end{gathered}$$

According to the question, 

$$\begin{gathered} \frac{116x}{100}+\frac{18480-22x}{25}=840 \\ \Rightarrow \frac{116x+73920-88x}{100}=840 \\ \Rightarrow 28x+73920=84,000 \\ \Rightarrow 28x=84,000-73,920 \\ \Rightarrow 28x=10,080 \\ \Rightarrow x=\frac{10080}{28} \\ \therefore x=360 \end{gathered}$$

The cost price of one watch is 360 Tk.

The cost price of another watch = (840-360)= 480 Tk. 

ans. The cost price of one watch is 360 Tk. & another watch is 480 Tk.