Molarity when 30g is dissolved in 100g of water = 0.5128moles/ dm^3
Molarity when 15g is dissolved in 100g of water= 0.2564moles/dm^3
Therefore the statement is not right, as the 15g has exactly half the molarity as the 30 g one. molarity only changes if the volume changes or the mass of the salt changes, it's given by the equation "number of moles X volume"