Question:medium

For any real number x, let [x] be the largest integer less than or equal to x. If \(\sum_{n=1}^N \left[ \frac{1}{5} + \frac{n}{25} \right] = 25\) then N is

Updated On: Jan 15, 2026
Show Solution

Correct Answer: 44

Solution and Explanation

Given the sum:

\[ \sum_{n=1}^N \left\lfloor \frac{1}{5} + \frac{n}{25} \right\rfloor = 25 \]

Simplify the expression within the floor function:

\[ \frac{1}{5} + \frac{n}{25} = \frac{5n + 1}{25} \]

The sum becomes:

\[ \sum_{n=1}^N \left\lfloor \frac{5n + 1}{25} \right\rfloor = 25 \]

For \( n = 1 \) to \( n = 19 \), the floor function evaluates to 0:

\[ \left\lfloor \frac{5n + 1}{25} \right\rfloor = 0 \quad \text{for } n \in [1, 19] \]

For \( n = 20 \) to \( n = 44 \), the floor function evaluates to 1:

\[ \left\lfloor \frac{5n + 1}{25} \right\rfloor = 1 \quad \text{for } n \in [20, 44] \]

Calculate the total sum:

\[ \sum_{n=1}^{N} \left\lfloor \frac{5n + 1}{25} \right\rfloor = \sum_{n=1}^{19} 0 + \sum_{n=20}^{44} 1 = 0 + (44 - 20 + 1) \times 1 = 0 + 25 = 25 \]

The value of \( N \) that satisfies the equation is:

\[ \boxed{N = 44} \]

Was this answer helpful?
0


Questions Asked in CAT exam