A man left (1/7)th of his property to his daughter and the remaining to his sons to be equally divided among them. If the share of the each son be double than that of the daughter. Then, find the number of sons?

Let the total property = N
Then, property of his daughter = P/7
∴ Remaining property = P - P/7 = 6P/7
Now, this property is divided among his sons, so that each son gets twice the property of his daughter.
Let number of sons be K.
Then, 6N/7 x K = 2N/7
⇒ K = 3