Find Initial Sum

Multiply the number of observations ($n = 5$) by the given mean ($15$).

$\text{Sum} = 5 \times 15 = 75$

Calculate New Sum

Subtract the removed observation ($25$) from the initial sum ($75$).

$\text{New Sum} = 75 - 25 = 50$

Find New Count

Subtract 1 from the total number of observations since one observation is removed.

$\text{New Count} = 5 - 1 = 4$

Compute New Mean

Divide the new sum by the new count of observations.

$\text{New Mean} = \frac{50}{4} = 12.5$