The class average x in a test increases by 4 when the score of a student is rectified, whose corrected score is 100 instead of 0. Later, the score of another student was found to have been recorded as 81 in place of 56. If there are no other corrections and the final corrected average is y, then y - x is<br/>

From the first correction, total marks increase by 100 (0 to 100). Since the average rises by 4, the number of students is 100/4 = 25.

The second statement says the score was "recorded as 81 in place of 56", i.e. 81 was entered instead of the correct 56. So the total must be reduced by 25.

Hence the average falls by 25/25 = 1 from the post-first-correction average.
So final average y = x + 4 - 1 = x + 3.
Therefore, y - x = 3.