Packing Intuition Math Example 1

Follow the full solution, then compare it with the other examples linked below.

Example 1

easy
Circular coins of radius 11 cm are packed into a 10โ€‰cmร—10โ€‰cm10\,\text{cm}\times 10\,\text{cm} square tray in a square grid arrangement. How many coins fit and what is the packing efficiency?

Solution

  1. 1
    Step 1: Each coin occupies a 2โ€‰cmร—2โ€‰cm2\,\text{cm}\times 2\,\text{cm} square cell. Number per row =10/2=5= 10/2 = 5. Total coins =5ร—5=25= 5 \times 5 = 25.
  2. 2
    Step 2: Area of each coin =ฯ€(1)2=ฯ€= \pi(1)^2 = \pi cm2^2. Total coin area =25ฯ€= 25\pi cm2^2.
  3. 3
    Step 3: Tray area =100= 100 cm2^2. Packing efficiency =25ฯ€100=ฯ€4โ‰ˆ78.5%= \dfrac{25\pi}{100} = \dfrac{\pi}{4} \approx 78.5\%.

Answer

2525 coins; packing efficiency โ‰ˆ78.5%\approx 78.5\%.
Square packing of equal circles achieves efficiency ฯ€/4โ‰ˆ78.5%\pi/4 \approx 78.5\%. About 21.5%21.5\% of space is wasted as gaps. Hexagonal (honeycomb) packing achieves โ‰ˆ90.7%\approx 90.7\%, which is why beehives use hexagons.

About Packing Intuition

Arranging objects of given shapes to fit as many as possible into a bounded region without any overlapping.

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