Pens are bought at 5 for ₹ 4 and sold at 4 for ₹ 5. Find the gain per cent.

Let the number of pens bought = 5 x 4 = 20
Then, CP = (4/5) x 20 = ₹ 16
and SP = (5/4) x 20 = ₹ 25
∴ Gain = 25 - 16 = ₹ 9
Gain % = (9/16) x 100% = 56.25%