The problem is asking for the largest jar that leaves the same amount of milk unmeasured from each vessel. This is equivalent to finding the greatest common divisor (GCD) of 67, 139, and 187.
We use the Euclidean algorithm to find the GCD:
1. Find \( \text{GCD}(67, 139) \):
\[
139 \div 67 = 2 \text{ (remainder 5)}, \quad 67 \div 5 = 13 \text{ (remainder 2)}, \quad 5 \div 2 = 2 \text{ (remainder 1)}, \quad 2 \div 1 = 2 \text{ (remainder 0)}.
\]
Thus, \( \text{GCD}(67, 139) = 1 \).
2. Now, find \( \text{GCD}(1, 187) \):
Since the GCD of any number and 1 is 1, we have:
\[
\text{GCD}(67, 139, 187) = 1.
\]
Therefore, the largest possible jar that can be used is 24 litres.