The given answer is incorrect, the minimum is one.
Weigh any 2 marbles against each other, there is a 1/7 chance of finding the odd one out on first try.

The answer given is to the question, “What is the minimum number of weighings required to guarantee finding the odd one out.”

Alternate (more efficient) answer: They use a box that can be opened if either of 2 locks are opened, the closed lock functioning as a hinge.