Answer:
Perth is located in the Australian Western Standard Time zone (AWST) and Sydney is in the Australian Eastern Standard Time zone (AEST). There is a time difference of 2 hours between the two cities, with Sydney being ahead.
To ensure the person in Perth contacts the business in Sydney before it closes at 5pm Sydney time, we need to account for the time difference. In Perth, they would need to call before 3pm Perth time (5pm Sydney time - 2-hour time difference).
Therefore, the person in Perth should phone the business in Sydney before 3pm local time in Perth to ensure they contact the business before it closes at 5pm Sydney time.