Question 1127387
To find out how quckly they can do it as a team, first we have to bring their individual results down to a common  metric so that we can compare apples with apples. So let's first bring their rates to one stack:
:
Worker 1: 1/30 (one stack/30 minutes)
Worker 2: 1/30
Worker 3: 1/60
1/30 + 1/30 + 1/60 = 1/minute 
multiply all times 60:
2t + 2t + 1t = 60
5t = 60
t = 12 minutes. This is how long it will take the three working together.