The average marks of 30 pupils in a test were worked out as 74. It was later found that two entries had been copied wrongly: 68 had been entered as 86, and 45 had been entered as 54. What is the correct average?

The recorded total is $30\times 74=2220$. Both entries were too HIGH, by 86 - 68 = 18 and by 54 - 45 = 9, a combined 27 marks too many. The true total is 2220 - 27 = 2193 and the correct average is 2193/30 = 73.1. 74.9 adds the 27 instead of subtracting it. 73.4 corrects only the first entry (74 - 18/30). 74.3 nets the two errors against each other as 18 - 9 = 9 and then adds it; both entries were overstated, so both corrections pull the same way.