Question

The average of 5 numbers is 12. Four of the numbers are 10,13,15 and 21. What is the fifth number

Answer 1

Let the fifth no. be x , than
\[\frac{10+13+15+21+x}{5}=12\]
Can you find x now?

Answer 2

(x+10+13+15+21)/5=12
x+59=12*5
x+59=60
x=1