If the price of pen decreases by 20%, then a man can buy 10 more pens for Rs 100. What is the new price (in Rs) of each pen?

let initial price = x

let he buy y number of pens for 100. $$\Rightarrow x\times y= 100$$.

If the price of pen decreases by 20%, then a man can buy 10 more pens for Rs 100

$$\Rightarrow (\frac{100-20}{100}\times x)\times (y+10)= 100$$

$$\Rightarrow (0.8\times x)\times (y+10)= 100$$

$$\Rightarrow (0.8x\times y)+(80\times x)= 100$$

$$\Rightarrow (80)+(80\times x)= 100$$

$$\Rightarrow (80\times x)= 100-80 = 20$$

$$\Rightarrow x= \frac{20}{8} = 2.5$$/-

new price = $$0.8\times x = 0.8\times 2.5 = 2$$/-

so the answer is option B.