The ratio of two number is $3:4$ and their LCM is $180$. The first number is

The correct option is B: $45$

Ratio of two numbers $=3:4$

Let the required no. be $3x$ and $4x$.

Then, their LCM will be $12x$.

But, LCM is given as $180$

$\therefore 12x=180$

$x=\frac{120}{12}=15$

Hence, the first no. $=3x=3\times 15=45$