Answer:
To understand this, bags of gas added make it go up, while bags of sand added make it go down:
1). Add bags of gas | balloon goes UP | 3 bags of gas ( +3 ), 10 bags of sand ( +10 )
2). Add bags of sand | balloon goes DOWN | 3 bags of sand ( -3 ), 10 bags of sand ( -10 )
3). Subtract bags of gas | balloon goes DOWN | subtract 3 bags of gas - ( +3 ), subtract 10 bags of gas - ( +10 )
4). Subtract bags of sand | balloon goes UP | subtract 3 bags of sand - ( -3 ), subtract 10 bags of sand - ( 10 )
HOPE THIS HELPS.