Let the number to be added is x
The numbers are: 15, -6, 12 and x
Sum = 15+(-6)+12+x = 21+x
Total numbers = 4
[tex]mean = \frac{sum}{total \: numbers} \\ \\ 5 = \frac{21 + x}{4} \\ \\ 5 \times 4 = 21 + x \\ \\ 20 = 21 + x \\ \\ x = 20 - 21 \\ \\ x = \boxed{ - 1}[/tex]
Number to be added is -1.
