A girl sold her pen for Rs.39 and got a percentage of profit numerically equal to the cost price. The cost price of that pen is:  Select:
A) 130  B) 90  C) 60  D) 30  E) 20
Let Cost Price was Rs. X then % profit must be X%.   Selling Price = Rs. 39   Cost price + profit = 39   X + X% of X = 39   X + (X^2)/100 = 39   100 X + X^2 = 3900   X^2 + 100X - 3900 = 0   X^2 +130X - 30X - 3900 = 0   X = -130, 30   Cost price cannot be negative so Cost Price was Rs. 30.   ANSWER : OPTION 30
<result>D</result>
A girl sold her pen for Rs.39 and got a percentage of profit numerically equal to the cost price. The cost price of that pen is: