If the cost price of 12 pens is equal to the selling price of 10 pens, then gain percent is:

A. 15%
B. 16%
C. 18%
D. 20%

Solution:
CP of 12 pens = SP of 10 pens
So, CP:SP = 10:12 = 5:6
Profit = 1 on CP=5 → %Gain = (1/5)×100 = 20%
Answer: D