Pipes cisterns work rate model – UGC NET Question

A is correct because one-hour work is the reciprocal of the total time needed. B is correct as an emptying pipe can be assigned a negative work rate. C is also correct since combined effect is the sum of filling and emptying rates with appropriate signs, and E is true because these problems are a direct application of time and work concepts. D is wrong because two filling pipes working together have a combined time less than either individual time; for 6 and 8 hours, the joint time is 24/7 hours, not 14 hours. Hence D only is the wrong statement.

Option A:
Option A correctly isolates D as the single incorrect statement, leaving the accurate descriptions of one-hour work, negative rates and time–work analogy intact. It reflects the usual modelling of pipes and cisterns problems in aptitude tests.

Option B:
Option B incorrectly treats both C and D as wrong, even though C correctly describes adding work rates with signs. By misclassifying C, this option fails the requirement to pick only wrong statements.

Option C:
Option C wrongly includes B as wrong along with D, despite B providing a standard way of modelling emptying actions with negative work. This misidentification makes the option invalid.

Option D:
Option D is incorrect because it also labels A as wrong, even though A properly defines one-hour work as the fraction 1/5. Thus it rejects a true statement while trying to catch the false one.