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Select the number that can replace the question mark (?) in the following series.
80, 79, 75, 66, ?, 25
Given series is 80, 79, 75, 66, ?, 25
The logic here is
80 - 79 = 1 = 1$$^2$$
79 - 75 = 4 = 2$$^2$$
75 - 66 = 9 = 3$$^2$$
The difference between the numbers of the series is the square of natural numbers.
The next difference will be 4$$^2$$ = 16.
Let the required number = a
$$\Rightarrow$$ 66 - a = 16
$$\Rightarrow$$ a = 50
$$\therefore\ $$Required number = 50
Hence, the correct answer is Option C
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