Let the other number be = y

Since,

\({Arithmetic Mean} = \frac{{Sum of numbers}}{{Total number of variables}}\)

or \(20 = \frac{{x + y}}{2}\)(since the number of variables is x and y i.e 2)

or \(x + y = 40\)

or \(y = 40 - x\)

So the other number in terms of x is = \(40 - x\)
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Theory: Average = \(\frac{Sum Of All The Values}{Total Number Of Values}\)

If the average (arithmetic mean) of two numbers is 20 and one of the numbers is x
Let other number be y
=> Sum of all the values= x + y

Using, Average = \(\frac{Sum Of All The Values}{Total Number Of Values}\), We get
20 = \(\frac{x+y}{2}\)
=> x + y = 20*2 = 40
=> y = 40-x

So, answer will be A
Hope it helps!

To learn more about Statistics watch the following video


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