Solution:Let the cost price of each pen be Rs. x
Cost price of 25 pens = 25x
Selling price = 25x × 1.2 = 30x
Cost price of 15 pens = 15x
Selling price = 15x × 1.1 = 16.5x
Total selling price = 46.5x
Profit = 6.5x
Now, cost price of 40 pens = 40x
Selling price = 40x × 1.15 = 46x
Profit = 6x
According to question,
6.5x - 6x = 64
or, x = Rs. 128