Suppose your given the following data: Country
| Literacy Rate (%) | Angola | 72.2 | Burundi | 73.3 | Democratic Republic of Congo | 70.4 | Rwanda | 77.6 | Kenya | 80.3 |
Mean: 
Median: 1) sort them  2) pick the one in the middle: 
Mode: since no numbers repeat, all numbers are the mode (or, rather, there is no mode)
Now let’s add one more country to the data Chad Literacy Rate = 37.6%
Repeat the exercise.
Now, our sample is

Mean

Median
Mode: same deal
You can think of Chad as an Outlier. Notice how much the mean changes -- it jumps from 74.76 down to 68.5666 -- while the median (the median has the advantage of being generally less susceptible to outliers) goes from 73.3 to 72.75.
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