On selling $18$ toys at $\text{Rs.}$ $840$, there is a loss equal to the cost price of $6$ toys. The cost price of $3$ toys is

The correct option is B: $\text{Rs.}$ $210$

Let the cost price of a toy be $\text{Rs.}$ $x$.

The cost price of $18$ toys $=$ $\text{Rs.}$ $18x$

The selling price of $18$ toys $=$ $\text{Rs.}$ $840$

Loss $=$ cost price of $6$ toys $=$ $\text{Rs.}$ $6x$

$\therefore$ Loss $=$ Cost price $-$ Selling price

$\Rightarrow 6x=18x-840$

$\Rightarrow 12x=840$

$\therefore x=\frac{840}{12}=70$

$\text{Hence, cost price of 3 toys}$ $=$ $\text{Rs.}$ $3\times 70=$ $\text{Rs.}$ $210$