Two solutions labelled as $3$M HCl and $1$M HCl are mixed in the ratio of $x:y$ by volume and the molarity of mixture solution becomes $1.5$ M. What is the molarity of the resulting solution if they are mixed in the ratio of $y:x$ by volume ?

The correct option is C $2.5$ M
Resultant molarity
$M=\frac{M_1{V}_1+M_2{V}_2}{V_1+V_2}$
Case I: when solution are mixed in the ratio of $x:y$ by volume
$1.5=\frac{3x+y}{x+y}$
or, $3x+y=1.5x+1.5y$
$1.5x=0.5y$
$\frac{x}{y}=\frac{1}{3}$
Case II: when solution are mixed in the ratio of $y:x$ means $3:1$.
$M=\frac{3\times 3+1\times 1}{1}=\frac{10}{4}=2.5$