make a proportion
Let x = the amount of feet descended in 3 minutes (thus the total change of altitude of the balloon)
320 (feet per minute) / 1 (minute) = x (feet per minute) / 3 (minutes)
1) Divide: 320/1 = 320.
2) Multiply: 320 x 3= 960.
The balloon that descends at a rate of 320 feet/minute (so, every minute, it descends 320 feet) descends a total of 960 feet in 3 minutes.
The balloon's total change in altitude is -960 feet.