Let the cost price of 1 pen be ₹x.
Step 1: Total cost price (CP)
For 10 pens:
[
CP = 10x
]
Step 2: Selling price (SP)
Given SP for 10 pens = ₹300
Step 3: Loss condition
Loss = cost price of 4 pens = (4x)
So,
Loss=CP−SP
4x=10x−300
10x−4x=300
6x=300
x=50
Step 4: Find CP and loss
Step 5: Loss percent
500200×100=40%