Given, mean of 10 observations is 2.5

Hence, sum of observations will be 2.5 x 10 = 25

Now, one observation namely 2.5 is deleted. So, sum of observations will be 25 – 2.5 = 22.5

And the number of observations will be 10 – 1 = 9

Therefore, new mean = 22.5 / 9 = 2.5