Six students sat a test marked out of 50 and their average was 42 marks. What is the least mark any one of them could have scored?

The six scored $6\times 42=252$ marks between them. One student's mark is as low as possible when the other five are as high as possible, and the paper is out of 50, so the other five can reach at most $5\times 50=250$. That leaves 252 - 250 = 2 marks for the sixth. 0 assumes the lowest score can always be zero, but then the other five would have to supply all 252 marks, which is 2 more than the cap of 250 allows. 42 assumes every other student sits exactly at the average, in which case nobody is below it at all. 8 is 50 - 42, the gap between the top mark and the average, which is not what limits the weakest student.