A product is sold at a profit of 20%. If the cost price is increased by 10% and sale price by Tk. 26, then the percentage of profit reduce by 5%, cost price is

Let, the cost price be Tk. x
Selling price at 20% profit
= x + 20% of x
= x + 20x/100
= 120x/100
= Tk. 6x/5

The cost price is increased by 10%.
So, new cost price
= (x + 10% of x)
= Tk. 11x/10

Therefore, new selling price = Tk. (6x/5 + 26)

New profit = (6x/5 + 26) - 11x/6
= Tk. (x/10 + 26)

According to the question,
(x/10 + 26) ÷ 11x/10 × 100 = 15
⇒ (x + 260) × 100/11x = 15
⇒ 65x = 26,000
∴ x = 400