Question:medium

How many three-digit numbers are greater than 100 and increase by 198 when the three digits are arranged in the reverse order?

Updated On: Jan 15, 2026
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Correct Answer: 70

Solution and Explanation

Analysis of the provided information:

Let the original three-digit number be \( ABC \), where A is the hundreds digit, B is the tens digit, and C is the units digit.

When the digits are reversed to form \( CBA \), the number increases by 198.

This translates to the equation:

\[ CBA = ABC + 198 \]

Expressing the numbers in terms of their digits:

\[ ABC = 100A + 10B + C \]

\[ CBA = 100C + 10B + A \]

Substituting these into the equation:

\[ 100C + 10B + A = 100A + 10B + C + 198 \]

Simplifying the equation by combining like terms:

\[ 99C - 99A = 198 \]

Dividing both sides by 99:

\[ C - A = 2 \]

Given that A and C are single-digit integers, the possible pairs for (A, C) are:

\[ (A, C) = (0, 2), (1, 3), (2, 4), (3, 5), (4, 6), (5, 7), (6, 8), (7, 9) \]

Since the number must be a three-digit number, the hundreds digit A cannot be 0. Therefore, the pair (0, 2) is invalid.

This leaves 7 valid combinations for (A, C).

The tens digit B can be any digit from 0 to 9. For each of the 7 valid (A, C) pairs, there are 10 possible values for B.

The total number of three-digit numbers satisfying the given conditions is:

\[ 7 \times 10 = 70 \]

Thus, there are 70 such three-digit numbers.

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