Final answer:
To find the new mean, add 4 to each observation and divide the new sum by the number of observations.
Step-by-step explanation:
To find the new mean, we first need to calculate the sum of the 25 observations. We know that the mean of these observations is 9, so the sum is 25 x 9 = 225.
If each observation is increased by 4, we can add 4 to each observation and calculate the new sum. The new sum is 225 + (4 x 25) = 325.
Finally, to find the new mean, we divide the new sum by the number of observations, which is still 25. So the new mean is 325 ÷ 25 = 13.
Learn more about Mean of observations