# Difference between revisions of "2015 AMC 10A Problems/Problem 14"

## Problem

Clock - radius 20 cm Disk - radius 10 cm, externally tangent at 12 o'clock, arrow pointing up

If the disk rolls clockwise, where on the clock will it be when the arrow points up?

## Solution

The circumference of the clock is twice that of the disk. So, a quarter way around the clock (3:00), the point halfway around the disk will be tangent. The arrow will point to the left. We can see the disk made a 75% rotation from 12 to 3, and 3 is 75% of 4, so it would make 100% rotation from 12 to 4. The answer is $\boxed{\textbf{(C) }4 }$ o'clock.