Question

A number of linked rings, each $1$ cm thick, are hanging on a peg. The top ring has an outside diameter of $20$ cm. The outside diameter of each of the other rings is $1$ cm less than that of the ring above it. The bottom ring has an outside diameter of $3$ cm. What is the distance, in cm, from the top of the top ring to the bottom of the bottom ring?

$171$

$173$

$182$

$188$

$210$

Answer: B

Concepts:arithmetic sequence summation

Difficulty rating: 1370

Solution:

The top ring spans $20$ cm. Each ring below overlaps the ring above by $2$ cm (twice the $1$ -cm thickness), so it adds its outside diameter minus $2.$

The lower rings have outside diameters $19, 18, \ldots, 3,$ contributing $17, 16, \ldots, 1.$ Thus the total distance is $\begin{gathered} 20 + (17 + 16 + \cdots + 1) \\ = 20 + \frac{17 \cdot 18}{2} \\ = 20 + 153 \\ = 173 \text{ cm}. \end{gathered}$

Thus, the correct answer is B.

2006 AMC 12A Problem 12

Answer: B

Difficulty rating: 1370

Solution:

Problem 12 in Other Years