Class A, max PAE 50%, 6dB back off means 1/4 of RF power, with same bias, DC power remain the same. So the efficiency become 50%/4.
Class B, biased on pinch off, so doesn't matter the lower RF power, it is still a class B, so the efficiency is still 78.5%.
This is an interview question I was asked, so it was not supposed to be very accurate or math intensive. It should be just applying some basic principle. The interviewer didn't say anything after I gave the answer, I am not sure if that was the right answer.