What number was removed?

A Basic GRE/GMAT Word problem:

The average of 8 numbers is 20.  If after one number is removed, the average of the remaining 7 numbers is 16.  What number was removed?

[Spoiler Below]

Remembering that the mean is the sum divided by the number of numbers:
1: `"Average"_8 = 20 = "Sum"_8/8`
2: `"Average"_7 = 16 = "Sum"_7/7`
so:
3: `160 = "Sum"_8`
and:
4: `112 = "Sum"_7`

noting that: `"Sum"_8 = "Sum"_7 + RemovedNumber`,
5: `"Sum"_8 = 160 = "Sum"_7 + RemovedNumber`


Subtracting (4) from (5), we get: `48 = RemovedNumber`

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