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

Let the number of students be n. The first correction adds 100 to the total (from 0 to 100), increasing the average by 100/n = 4, so n = 25. The second correction changes 81 to 56, decreasing the total by 25, so the average decreases by 25/25 = 1. Starting from original average x: after first correction average = x + 4. After second correction y = x + 4 - 1 = x + 3. Therefore y - x = 3. Wait — re-reading: the first correction increases x by 4, so the new average after first correction is x+4. Then the second correction changes 81 (recorded) to 56 (actual), meaning total decreases by 25, so average changes by -1. Final y = (x+4) - 1 = x+3. So y-x = 3.