Option 3 : Rs. 150

The cost price of the article is

⇒ CP = (21/14) × 100

⇒ CP = (3/2) × 100

⇒ CP = Rs. 150

∴ The cost price is Rs. 150.

**Given:**

The difference between the two selling price of an article = Rs. 21,

An article is sold at 7% loss and another at 7% profit.

**Formula used:**

Selling price in case of profit

⇒ SP = {(100 + P%)/100} × CP

Selling price in case of loss

⇒ SP = {(100 - L%)/100} × CP

where,

SP → Selling price, CP → Cost price,

P% → Profit percentage, L% → Loss percentage

**Calculations:**

Let the cost price of an article be y

According to the question, we have

The selling price at 7% profit

⇒ SP_{1} = {(100 + P%)/100} × CP

⇒ SP_{1} = {(100 + 7%)/100} × y

⇒ SP_{1} = (107/100) × y

⇒ SP_{1} = 1.07y

Selling price in case of loss

⇒ SP_{2} = {(100 - L%)/100} × CP

⇒ SP_{2} = {(100 - 7%)/100} × y

⇒ SP_{2} = (93/100) × y

⇒ SP_{2} = 0.93y

Now, difference between the two selling prices is Rs. 21

⇒ SP_{1} - SP _{2} = 21

⇒ 1.07y - 0.93y = 21

⇒ 0.14y = 21

⇒ y = 21/0.14

⇒ y = Rs. 150

**∴ The cost price is Rs. 150.**