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XYZ is a 3-digit number, where X, Y, Z are distinct non-zero digits. The difference between the two 3-digit numbers XYZ and YXZ is 90. How many possible values exist for the sum (X+Y) ?

This question was previously asked in
CDS 2 2024 Maths Question Paper (01-Sep-2024)
The correct answer is
8

To solve the problem, we need to determine how many possible values exist for the sum (X + Y), given that the difference between the two 3-digit numbers XYZ and YXZ is 90.

Let's represent the 3-digit numbers:

  • XYZ can be expressed as 100X + 10Y + Z.
  • YXZ can be expressed as 100Y + 10X + Z.

The problem states:

  • (100X + 10Y + Z) - (100Y + 10X + Z) = 90

Simplifying this equation:

  • 100X + 10Y + Z - 100Y - 10X - Z = 90
  • (100X - 10X) + (10Y - 100Y) = 90
  • 90X - 90Y = 90
  • 90(X - Y) = 90

Divide both sides by 90:

  • X - Y = 1

This tells us that the digits X and Y are consecutive, with X being one more than Y.

Since X and Y are distinct and non-zero, we calculate the possible pairs:

  • Y = 1, X = 2
  • Y = 2, X = 3
  • Y = 3, X = 4
  • Y = 4, X = 5
  • Y = 5, X = 6
  • Y = 6, X = 7
  • Y = 7, X = 8
  • Y = 8, X = 9

For each pair, we calculate X + Y:

  • For (X, Y) = (2, 1), X + Y = 3
  • For (X, Y) = (3, 2), X + Y = 5
  • For (X, Y) = (4, 3), X + Y = 7
  • For (X, Y) = (5, 4), X + Y = 9
  • For (X, Y) = (6, 5), X + Y = 11
  • For (X, Y) = (7, 6), X + Y = 13
  • For (X, Y) = (8, 7), X + Y = 15
  • For (X, Y) = (9, 8), X + Y = 17

The possible sums are 3, 5, 7, 9, 11, 13, 15, 17, providing us with 8 distinct possible sums.

Thus, the number of possible values for (X + Y) is 8.

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