Egghead Games

How to Solve Logic Grid Puzzles

A logic grid puzzle gives you a short story, a few clues and a grid. Everything you need is in the clues: there is no guessing and no outside knowledge. This guide solves one puzzle from start to finish and explains each kind of clue along the way.

The puzzle

Butterfly Auction

An auction was held today at which a number of rare butterfly specimens were sold. Determine the price and order of each auction lot, as well as the name of each winning bidder.

  1. The insect that was auctioned 3rd was either the insect won by Joel or the butterfly that sold for $250.
  2. The insect that sold for $280 was the 9th lot.
  3. Of the butterfly that sold for $260 and the butterfly that was auctioned 4th, one was won by Harry and the other was won by Wallace.
  4. The insect that was auctioned 4th sold for 10 dollars less than the insect that was auctioned 6th.
  5. The butterfly won by Marco didn’t sell for $270.
  6. The five butterflies were the butterfly that was auctioned 3rd, the insect that sold for $260, the butterfly that sold for $270, the butterfly that sold for $280 and the butterfly won by Harry.

You can play this puzzle yourself. It is in the free volume of our Logic Grid Puzzles app, under the title Lepidopterists, which means people who collect butterflies.

How the grid works

The puzzle has three categories (prices, winners and orders) with five items in each. The orders are the points in the auction at which each butterfly came up: 2nd, 3rd, 4th, 6th and 9th. Solving the puzzle means working out who won each butterfly, what it sold for, and when it came up.

Butterfly Streamline Illustration: https://streamlinehq.com $250$260$270$280$2902nd3rd4th6th9thHarryJoelKellyMarcoWallace2nd3rd4th6th9thWINNERSORDERSPRICESORDERS
The empty grid, as it looks in the app.

The grid is really three small grids joined together. Each one compares two of the categories.

1 2 3
Grid 1 compares prices with winners, 2 compares prices with orders, and 3 compares orders with winners.

Orders appear twice, once across the top and once down the side, so that every pair of categories gets a grid of its own.

Each square is a yes-or-no question about two items.

This square asks: did Kelly win the $260 butterfly?

You answer each question with one of two marks: a green circle for yes, or a red X for no.

The two rules

Every logic grid puzzle follows the same two rules.

  1. Everything has exactly one match in each category. Each butterfly sold for one price, to one winner, at one point in the auction.
  2. Nothing is shared. No two butterflies had the same price, the same winner or the same place in the order.

On the grid, this means each of the three small grids ends up with exactly one circle in every row and every column, and an X everywhere else. When all three are full, the puzzle is solved.

Solving it, clue by clue

Each part below takes one kind of clue and shows what to mark on the grid. They follow on from each other, so by the end of the last part the puzzle is solved.

  1. True and false clues
    Clues that state a fact outright, and the X's that follow from them.
  2. Either/or clues
    Clues that offer two options, exactly one of which is true.
  3. Greater than clues
    Clues that compare two items, such as "10 dollars less than".
  4. Pair clues
    Clues that read "of A and B, one is X and the other is Y".
  5. All-different clues
    Clues that list several items and say they are all different.
  6. Finishing the grid
    What to do when you have read every clue and the grid still isn't full.

Going further

Most puzzles can be finished with the six parts above. Harder ones sometimes need a second look at the clues, or at the grid itself. Each of these parts shows one technique on a real puzzle, part-way through.

  1. A second look at either/or and pair clues
    Combining an either/or or pair clue with what the grid already shows.
  2. Combining greater-than clues
    Comparing two greater-than clues that mention the same item.
  3. No options in common
    Two items with no remaining option in common can't go together.
  4. Locked squares
    When one of two items must take a square, nothing else can.
  5. When you're stuck
    A checklist, note-taking, and trial and error as a last resort.

Try one yourself

You can play puzzles like this one in Logic Grid Puzzles, our app for phones and tablets. It fills in the X's around each match for you, lets you undo as far back as you like, and offers a hint when you get stuck.