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The figure shows two 4-tile patterns.

Two 4-tile patterns: a 2 by 2 square of tiles, and a straight line of 4 tiles

Either one or both of the patterns can be used any number of times and in any orientation to construct a new pattern. Which one of the options below cannot be constructed by using only these two 4-tile patterns assuming there are no overlaps among them?

  • A.A rectangle of tiles, 2 rows by 4 columns
  • B.A rectangle of tiles, 3 rows by 4 columns
  • C.A rectangle of tiles, 3 rows by 5 columns
  • D.A rectangle of tiles, 4 rows by 5 columns

Answer: C

The pattern that cannot be built is (C), the 3 by 5 rectangle.

Both building blocks contain exactly 4 tiles, and pieces may not overlap, so any pattern made only from them must contain a multiple of 4 tiles. Count the tiles in each option:

  • (A) 2 × 4 = 8 tiles: two 2 × 2 squares side by side, or two lines of 4 stacked.
  • (B) 3 × 4 = 12 tiles: three horizontal lines of 4.
  • (C) 3 × 5 = 15 tiles: not a multiple of 4.
  • (D) 4 × 5 = 20 tiles: five vertical lines of 4.

Since 15 is not divisible by 4, no combination of the two patterns can cover the 3 by 5 rectangle exactly, while each of the others is built as shown.

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