• A student computed the mean of a particular sample to be 40. After computing the mean, he discovered that he forgot to include the number 36 to the sample. When this number was included. The sample mean changed to 39.5. What is the sample size (n) including the number 36?
If the mean was 40 and the sample size was 1. Adding 36 would decrease the mean by 2. to 38

But here the mean decreased by 0.5, which is 4 times smaller than was observed. So using ratios the true sample size
is 2x4 = 8.