Answer: 30 minutes.
Explanation:
Given : Brett, Lucy, and Donnie are each making trips carrying bags of mulch from the back losing dock to the front of the store.
It takes Brett 3 minutes to make a trip , Lucy 5 minutes to make a trip and Donnie 6 minutes to make a trip.
Then , the number of minutes at which they all arrive back at the same time= Least common multiple of 3,5, and 6 i.e. LCM(3, 5, 6)
Prime factorization :-
![3=3*1\\5=5*1\\6=3*2*1](https://img.qammunity.org/2020/formulas/mathematics/middle-school/147gw71a4sgi0yrvrx0axwapl36tnif1e3.png)
Then,
![\text{LCM(3, 5, 6)}=3*2*5=30](https://img.qammunity.org/2020/formulas/mathematics/middle-school/2l0ob2n8as2w647eclheptkostq77l11ew.png)
Hence, they all arrive after 30 minutes at the time.