You're told that the hot air balloon lowers itself (descends) by 320 feet a minute. That means for each minute that passes, the altitude changes by 320 feet.
Since you want to find the total change in altitude in 3 minutes, you would need to multiply 320 feet/min by 3 minutes. (Remember multiplication is the same thing as repeated addition, and you're repeatedly adding a change in 320 feet for every minute that passes):
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Answer: The total change in altitude is 960 feet.