19 trivia questions only geniuses will get right

Name the number that is three more than one-fifth of one-tenth of one-half of 5,000.

A. 503
B. 103
C. 53
D. 108

Work backward! Half of 5,000 is 2,500. One-tenth of that is 250. One-fifth of that is 50. Add three, and you’ve got your answer.

What’s the missing number?

A. 20
B. 21
C. 25
D. 17

Each number is the previous two numbers added together. The eighth number is the sixth and seventh numbers – 8 and 13 – added together. If you can figure this out, see if you can solve this puzzle dubbed the hardest ever by a university professor.

A. Istanbul, Turkey
B. Athens, Greece
C. Jerusalem
D. Damascus, Syria

Evidence of civilisation in Damascus dates all the way back to 9000 BC.

Two people are standing back to back. They each walk away from each other for three feet. Then they both turn left and walk for another four feet, and then stop. Now, how many feet apart are they standing?

A. 10
B. 7
C. 25
D. 5

If you remember the a² + b² = c² rule from maths class, that’s what’ll help you solve this problem. This rule states that if you have a triangle, the sum of the squares of the two shorter sides equals the square of the longest side. And in this problem, the walkers’ paths form parts of triangles. You may want a pencil and paper to “draw out” this problem and visualize the triangles.

Draw two lines labelled “three feet” for the distance they walk away from each other. Then draw two lines labelled “four feet,” going in opposite directions, for the distance they walked after their left turns. Now draw a line connecting the points at the ends of those lines (representing where the people are now). This line represents the distance you’re trying to figure out.

Now, you’ve got two triangles touching at the corners. Two sides of each are 3 feet and 4 feet (the distances each person walked). The unknown sides represent two halves of the distance you’re trying to find. So break out that Pythagorean Theorem: Three is a, 4 is b. 3² + 4² = 9 + 16 = 25 = c². Take the square root of 25 and you get 5, which is the longest side of these mini-triangles. Five feet is half of the distance between the people. Five times two is ten! Here’s another three-sided puzzler: try to figure out how many triangles are in this image.

You’re trapped in a room with two doors. Only one door will lead you out of the room safely, but you don’t know which. A guard stands in front of each door. One guard always lies, the other always tells the truth, but you don’t know which is which. You can only ask one guard one question. What question do you ask, and what do you do once the guard has answered?

A. “Which is the safe door?” Go through the door the guard tells you.
B. “Which is the safe door?” Go through the other door.
C. “If I were to ask the other guard which was the safe door, which door would s/he say?” Go through that door.
D. “If I were to ask the other guard which was the safe door, which door would s/he say?” Go through the other door.

If you chose the lying guard, the lying guard is telling you the door that the truthful guard would not say is safe. If you chose the truthful guard, the truthful guard is telling you the door that the lying guard would say is safe. Either way, the door the guard responds with is not the safe door. Go through the other door and you’re out! If you got that one right, you might be ready to solve Einstein’s riddle, which only 2 per cent of people can solve.