Since the question doesn't specify any other temperature for the 50 g of water we assume that that sample is at 70 degrees Celsius. Now simply find the amount of KCl that can dissolve in 50 g of water. Let this value be x. Let x/50 = 40/100 since the ratios are the same. Multiply 50 to both sides to get x = 2000/100 = 20. So 20 g of KCl can be dissolved in 50 g of water at 70 degrees Celsius.