The cost of ball pen is Rs 5 less than half of the cost of fountain pen.

Let the cost of fountain pen be y and cost of ball pen be x.

According to the given equation, we have

x = y/2 − 5

⟹ 2x = y - 10

⟹ 2x - y + 10 = 0

Here y is the cost of one fountain pen and x is that of one ball pen.