A man purchases as certain number of oranges at 4 a rupee and the same number of orange at 5 a rupees. He mixes them together and sells them at 4 a rupee. What is the gain or loss percent?

CP of 1 orange for 1st rate = ₹ 1/4
CP of 1 orange for 2nd rate ₹ 1/5
CP of 2 orange after mixing = 1/4 + 1/5 = ₹ 9/20.
CP of 1 orange after mixing ₹ 9/20 X 2 = ₹ 9/40
SP Of 1 orange after mixing = ₹ 1/4
∴ Gain = 1/4 - 9/40 = ₹ 1/40
∴ Gain = (1/40 x 40/9 x 100)% = 100/9%
= 11¹/₉%