Two Glass Cards
Read the puzzle
Two glass cards each have two seven-segment digit spaces with only some strokes lit. Card A: left digit lit at the two upper side strokes; right digit lit at the top, both upper sides and the middle. Card B: left digit lit at the middle and lower right; right digit lit at the middle, both lower sides and the bottom. An example shows two partial cards stacking to make 3. Slide the two cards together. Which number appears?
Slide the two glass cards together. Which number appears?
Imagine the strokes of both cards on one card.
48
Card A's strokes fill the gaps on card B: 4 and 8.
How to solve it
Each digit space has seven strokes: top, upper left, upper right, middle, lower left, lower right and bottom. Stacking clear cards shows every stroke lit on either card.
Step one: combine the left digits. Card A gives upper left and upper right. Card B gives middle and lower right. Together that is upper left, upper right, middle and lower right, which is the shape of a 4.
Step two: combine the right digits. Card A gives top, upper left, upper right and middle. Card B gives middle, lower left, lower right and bottom. Together all seven strokes are lit, which is an 8.
So the number that appears is 48.
The usual wrong answers come from reading each card on its own. Card A looks like 11 next to a small raised 0, and card B looks like 70, so people answer 110 or 70. Neither card shows the real number until its strokes are added to the other card's.
Quick check: the example works the same way. Top, upper right and middle on one card, plus lower right and bottom on the other, make a 3.
