Answer:
$55
Explanation:
Opportunity cost refers to the value of benefit forgone, in order to get the benefit of current option chosen.
Here, Cameron has a gift card worth $40 which he had to use,
Now he had two options:
Either to buy keyboard of $50
or
To buy speakers costing $55
He chooses to buy keyboard and the value of benefit forgone is value of speakers = $55.
Thus, opportunity cost = $55