BrightKidz Library
Subjects
A logic grid being solved by ruling things out A grid with a row of outline shapes along the top and a column of solid shapes down the left. Most of the squares inside hold a cross, meaning that pairing has been ruled out. Three squares hold a tick instead. A single large tick stands on the right, for the one answer left standing.

Solving Logic Grid Puzzles

About 6 minutes

A logic grid puzzle gives you a handful of clues and never tells you an answer directly. You get there by ruling things out — and the surprising part is that ruling enough things out leaves exactly one possibility standing.

The grid is just somewhere to keep your crossings-out so you do not have to hold them all in your head.

The Puzzle

Four friends — Alice, Bob, Carol and David — each own a dragon. The dragons are red, blue, green and yellow, one each.

The clues:

  1. Alice's dragon is not red and not blue.
  2. Bob's dragon is not green.
  3. Carol owns the red dragon.
  4. David's dragon is not blue and not yellow.

Setting Up

Draw a grid with the four names down the side and the four colors across the top. Every square is a question: does this person own this color? You will write ✗ for no and ✓ for yes.

A blank logic grid for the dragon puzzle A grid with Alice, Bob, Carol and David down the left side and Red, Blue, Green and Yellow across the top. Every square is a question: does this person own this colour of dragon? Mark each one with a tick or a cross. Red Blue Green Yellow Alice Bob Carol David
Copy this onto paper. Every square is one question, answered with a tick or a cross.

Solving It

  1. Clue 3 first, because it is the only one that says yes. Put ✓ where Carol meets Red. Now fill Carol's whole row with ✗ (she cannot own a second dragon) and the whole Red column with ✗ (nobody else can own the red one).
  2. Clue 1. Alice is not red — already crossed out — and not blue. Cross Alice/Blue.
  3. Clue 4. David is not blue and not yellow. Cross both.
  4. Look at David's row. Red is gone from step 1, blue and yellow from step 3. Only Green is left, so David owns green. ✓ it, then cross out the rest of the Green column.
  5. Look at Alice's row. Red, blue and now green are all crossed out. Alice owns yellow.
  6. Bob gets what is left, which is blue. Check it against clue 2 — Bob is not green, and he is not — so nothing is broken.

The order matters. Starting with clue 3 is what makes the rest fall over, because a single ✓ lets you cross out a whole row and a whole column at once.

Which dragon does Alice own?

The Answer

Carol has red, David has green, Alice has yellow and Bob has blue.

Notice that clue 2 was never really needed to find the answer — David's row solved itself from clues 1, 3 and 4. Clue 2 is there to let you check your work. Good puzzles often include one clue like that.

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