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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