CP of pen = ₹20
Profit % = 20%
Profit % = profit × 100 / CP
20 = profit × 100 / 20
20 × 20 / 100 = profit
Profit = ₹4
Profit = SP - CP
4 = SP - 20
SP = ₹24
Let new CP be ₹x
Profit % = 25%
SP = ₹24
Profit % = SP - CP
25x / 100 = 24 - x
24x / 100 + x = 24
125x / 100 = 24
5x / 4 = 24
x = 24 × 4 / 5
x = ₹ 19.20