Say I have 100 numbers that are averaged:

```
number of values = 100
total sum of values = 2000
mean = 2000 / 100 => 20
```

If I want to add a value and find out the new average:

```
total sum of values = 2000 + 100
mean = 2100 / 101 => 20.79
```

If I want to subtract a value and find out the new average:

```
total sum of values = 2100 - 100
mean = 2000 / 100 => 20
```

It seems to work, but is the above correct?

Is this the proper way to add/subtract values from a average without having to re-sum all the 100 numbers first?