A merchant sells 30 metres of cloth and gains a selling price of 10 metres. Find the gain percent.

Here,
The selling price of 10 m cloth is obtained as profit.
Profit of 10 m cloth = (S.P. of 30 m cloth) – (C.P. of 30 m cloth)
The selling price of 20 m cloth = Selling Price of 30 m of cloth

Let the cost of each metre be Tk. 100.

Therefore,
the cost price of 20 m cloth = 20 × 100 = Tk. 2000 and
S.P. of 20 m cloth = Tk. 3000

Profit% = (10/20) × 100
= 50%