Two chairs and a table can be bought for $\text{Rs. }170$. Five chairs and four tables can be bought for $\text{Rs. }530$. Find the cost of a chair.

Let the cost of one chair be $c$ and the cost of one table be $t$.
According to the first condition:

$$2c+t=170⟹t=170-2c\text{— (Equation 1)}$$

According to the second condition:

$$5c+4t=530\text{— (Equation 2)}$$

Substitute the value of $t$ from Equation 1 into Equation 2:

$$5c+4(170-2c)=530$$

$$5c+680-8c=530$$

$$-3c+680=530$$

$$-3c=530-680$$

$$-3c=-150⟹c=50$$

Thus, the cost of a chair is $\text{Rs. }50$.