Let the four numbers be:

5, 9, 12, and x

Given the average of the four number is 9.

We know that the average:

Av. = (sum of all numbers)/ total number of numbers

==> 9 = (5+9+12+x)/4

==> 9 = (26+ x)/4

Multiply by 4:

==> 36 = 26 + x

==> x= 36- 26= 10

**==> the fourth number = 10**

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