Correct Answer - Option 3 : 47
Concept:
\(\rm Mean\ of\ 'n'\ observations=\dfrac{Sum\ of\ observations}{n}\).
Calculation:
Let's say that the third number is x.
So, Numbers are 4, 12 and x
Here n = 3
As we know, \(\rm Mean\ of\ 'n'\ observations=\dfrac{Sum\ of\ observations}{n}\)
∴ \(\rm 21=\dfrac{4+12+\rm x}{3}\)
⇒ x + 16 = 63
⇒ x = 47.