# Data Structures | Stack | Question 7

Let S be a stack of size n >= 1. Starting with the empty stack, suppose we push the first n natural numbers in sequence, and then perform n pop operations. Assume that Push and Pop operation take X seconds each, and Y seconds elapse between the end of one such stack operation and the start of the next operation. For m >= 1, define the stack-life of m as the time elapsed from the end of Push(m) to the start of the pop operation that removes m from S. The average stack-life of an element of this stack is**(A)** n(X+ Y)**(B)** 3Y + 2X**(C)** n(X + Y)-X**(D)** Y + 2X**Answer:** **(C)****Explanation:** We can easily arrive at the result by taking few examples.

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