A clock's pendulum is replaced by a spring-mass system featuring a...

The time period of a standard clock pendulum (seconds pendulum) is $T = 2 \text{ s}$.

The time period of a spring-mass system is given by $T = 2\pi \sqrt{\dfrac{m}{k}}$.

Equating the two time periods:

$2 = 2\pi \sqrt{\dfrac{m}{0.1}}$

$1 = \pi \sqrt{\dfrac{m}{0.1}}$

Squaring both sides and using the approximation $\pi^2 \approx 10$:

$1 = 10 \left(\dfrac{m}{0.1}\right)<br><br>$1 = 100 m$<br><br>$m = $\dfrac{1}{100} \text{ kg}<br><br>$m = 0.01 \text{ kg} = 10 \text{ g}