What amount is entered in the adjusted cost base. I don't understand the explanation. Is it the amount paid at time of purchase of amount left over after selling.

The ACB of an asset is the price you paid to acquire it. Additionally, you can usually include capital costs such as the cost of additions or improvements and legal fees as part of an asset’s ACB. 

For full information view link below:

https://turbotax.intuit.ca/tips/calculating-adjusted-cost-base-6285

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