The solubility of KCl in water is 34.2 g per 100 g of water.
That means at STP, 34.2 g of KCl is dissolved in 100 g of water.
So, 100 g of water dissolved= 34.2 g of KCl
1 g of water dissolved=\frac{34.2 g of KCl}{100}
200 g of water dissolved=\frac{34.2 g of KCl}{100}\times 200
=68.4 g of KCl
So, the maximum amount of KCl that can dissolve in 200 g of water is 68.4 g.