Assuming all of his property was ' _crowns_ ', let the total amount he had be $\mathbf{T}$ crowns. Now the first son gets $\mathbf{100}$ crowns, so $\mathbf{T-100}$ crowns are left, of which he gets one-tenth, so his total inheritance is $\mathbf{\frac{T-100}{10} + 100}$ crowns. The second son first takes $\mathbf{200}$ crowns from the remaining estate, leaving us with $\mathbf{T-(200 + 100 + \frac{T-100}{10}) = \frac{9T - 2900}{10}}$ crowns, of which he takes one-tenth again. Gievn that each son receives an equal share, we equate these amounts with $\mathbf T$ being unknown $$\mathbf{\frac{T-100}{10} + 100 = \frac{9T - 2900}{100} + 200}$$ On solving, you are left with $\mathbf{T = 8100}$, so each son receives $\mathbf{900}$ crowns, i.e. there are $\mathbf9$ sons