Answer
$L=8.9m$
Work Step by Step
To find the period, note that if it takes 60 seconds to complete 10 oscillations; the period is (60s)/(10oscillations)=6.0 seconds. Note that the period of a pendulum is equal to $$T=2\pi \sqrt{\frac{L}{g}}$$ Solve for $L$ to get $$\frac{T}{2\pi}=\sqrt{\frac{L}{g}}$$ $$\frac{T^2}{4\pi^2}=\frac{L}{g}$$ $$L=\frac{T^2g}{4\pi^2}$$ Substituting known values of $T=6.0s$ and $g=9.8m/s^2$ yields a length of $$L=\frac{(6.0s)^2(9.8m/s^2)}{4\pi^2}=8.9m$$
