If a man gains 10% by selling a watch for Taka 660, find the cost price.

Question: If a man gains 10% by selling a watch for Taka 660, find the cost price.

Answer: Selling Price = 110% of Cost Price
→ Cost Price = 660 ÷ 1.1 = 600

Another Method,
With 10% gain,
Selling Price is 110 Taka when Cost Price is 100 Taka
Selling Price is 1 Taka when Cost Price is 100/110 Taka
Selling Price is 660 Taka when Cost Price is (100×660)/110 Taka = 600 Taka