Theory

➡ Average = Sum of all the Values / Total Number of Values
➡ Sum of All the values = Average * Total Number of Values

The average (arithmetic mean) of 10, 30, and 50 is 5 more than the average of 20, 40, and

Let the unknown number be x

Average of 10, 30, and 50 = \(\frac{Sum}{3}\) = \(\frac{10+30+50}{3}\) = \(\frac{90}{3}\)

Average of 20, 40 and x = \(\frac{Sum}{3}\) = \(\frac{20+40+x}{3}\) = \(\frac{60+x}{3}\)

Average of 20, 60 and 180 = 5 + Average of 10, 30 and x (given)

=> \(\frac{90}{3}\) = 5 + \(\frac{60+x}{3}\)
=> 90 = 5*3 + 60 + x = 15 + 60 + x = 75 + x
=> x = 90 - 75 = 15

So, Answer will be A.
Hope it helps!

Watch the following video to Learn the Basics of Statistics