21 pencils and 29 pens cost Rs. 79. But, if the number of pencils and pens were interchanged, the cost would have reduced by Rs. 8. Find the cost of each pen.

Let's denote the cost of a pencil as 'p' and the cost of a pen as 'q'.

From the first statement, we can form the equation: 21p + 29q = 79

From the second statement, we can form the equation: 29p + 21q = 79 - 8 = 71

Now we have a system of two equations, and we can solve it step by step.

Step 1: Multiply the first equation by 29 and the second by 21, to make the coefficients of p the same in both equations:

2921p + 2929q = 7929 2129p + 2121q = 7121

This gives us:

609p + 841q = 2291 609p + 441q = 1491

Step 2: Subtract the second equation from the first to eliminate p:

841q - 441q = 2291 - 1491 400q = 800

Step 3: Solve for q:

q = 800 / 400 = 2

So, each pen costs Rs. 2.