Question

What number must b added to the set 9,17,21 and 13 to get average of 15?

Answer 1

(9+17+21+13+x)/5=15
60+x=75
x=15

Answer 2

let we added x
so that
$\frac{9 + 17 +21 + 13 + x }{5} = 15 \\ = \frac{60 + x}{5} = 15 \\ 60 + x = 15 \times 5 \\ 60 + x = 75 \\ x = 75 - 60 \\ x = 15$
answer is 15