How do I determine the useful life of the Speaker from the given information and will the salvage value be 0 in ten years?

For example: Forecasts from the accounting department indicate that each special Speaker will be worthless 10 years from today. Does the 10 years in this example indicate the useful life?

Also another example: according to the ATO, special Speakers (used in the stadium) have an effective life of 8 years. Does the 8 years refer to the useful life?

Also, second question relating to first, how would I calculate the Written down value if the stadium uses the Speakers for 10 years and from the given information below:

The stadium has two idle Speakers that were purchased nine years ago for $3 million each and both Speakers have been written off for tax purposes. Forecasts from the accounting department indicate that each special Speaker will be worthless 10 years from today. According to the International Stadium Authority each special Speaker currently has a market value of $1,440,000.

According to the ATO, special Speakers (used in the stadium) have an effective life of 8 years.