Calculate the difference for each re-evaluated subject

Find the change in marks for each of the three subjects where marks were modified by subtracting the original marks from the new marks: $89 - 75 = 14$, $90 - 82 = 8$, and $88 - 80 = 8$.

$\Delta_1 = 14, \Delta_2 = 8, \Delta_3 = 8$

Find the net total change in marks

Sum up all individual mark changes to find the total increase in marks across all subjects: $14 + 8 + 8 = 30$.

$\text{Total} \, \text{Increase} = 14 + 8 + 8 = 30$

Calculate the increase in average marks

Divide the net total increase in marks by the total number of subjects ($n = 6$) to find how much the average increases: $\frac{30}{6} = 5$.

$\text{Average} \, \text{Increase} = \frac{30}{6} = 5$

Determine the new average marks

Add the average increase to the old average marks: $94 + 5 = 99$.

$\text{New} \, \text{Average} = 94 + 5 = 99$