Unitary method: If 12 pens cost ₹ 84, what is the cost of 10 such pens?

Introduction / Context:
The unitary method helps convert a given bundle price to a per-item rate, then to the cost of another quantity. This is routine in direct proportion pricing problems.

Given Data / Assumptions:

• 12 pens → ₹ 84 total.
• Find cost for 10 pens at the same unit rate.

Concept / Approach:
Compute the unit price and scale by the desired count: cost = (total/quantity) × target quantity.

Step-by-Step Solution:

Verification / Alternative check:
Proportional check: 10 is 5/6 of 12, so cost should be 5/6 of ₹ 84 → ₹ 70. This matches perfectly.

Why Other Options Are Wrong:

• ₹ 72 or ₹ 90: These correspond to different counts at ₹ 7 each (e.g., 10.285… pens or nearly 13 pens), not 10.
• ₹ 65: Not consistent with the computed unit price.

Common Pitfalls:
Rounding the unit price prematurely (e.g., calling it ₹ 7.1) or dividing the wrong way. Always verify the unit price first, then multiply.

Final Answer: