Answer:
you have to figure out who blow one baloon in least time
you can see in this case is Vishal
Explanation:
Ethan: 15 ballons ... 20 min
1 baloon ... x min x= 20*1 / 15 = 1,333
JunWei: 18 baloons ... 25 min
1 baloon ... x min x= 1*25 / 18 = 1,388
Vishal: 16 baloons ... 21 min
1 baloons ... x min x= 1*21 / 16 = 1,312