Step-by-step explanation:
We have
A simple pendulum is observed to swing through 71 complete oscillations in a time period of 1.80 min.
The frequency of a pendulum is equal to the no of oscillation per unit time. so,

Tim period is reciprocal of frequency. So,

The time period of a pendulum is given by :

l is length of pendulum

So, the period and length of the pendulum are 1.53 s and 0.58 m respectively.