Kevin Langdon Omni IQ test
Q56
You are
given:
 a 2½gallon container full of water;

an empty 1gallon container;
 a 1½pound weight;
 a 2½pound weight;
 a 4½pound weight;
 a 6½pound weight; and
 a 2pan balance.
Each container weighs five pounds. A pint of
water weighs one pound. One of the weights is slightly inaccurate  either
lighter or heavier than the weight indicated above. Which weight is
inaccurate and in which direction:
A. can be determined in three weighings.
B. can be determined in four weighings.
C. can be determined as to which weight in two weighings, but which direction it is off cannot necessarily be determined.
D. can be determined as to which weight in three weighings, but which direction it is off cannot necessarily be determined.
E. cannot be determined from the given conditions.