Hard Riddles Quiz
Questions: 16 · 10 minutes
1. Without altering either number, in what familiar setting does adding 5 to 9 produce 2?
On a standard calculator
On a number line using ordinary addition
In a base-ten multiplication table
On a 12-hour clock
2. Nine identical-looking coins include exactly one lighter counterfeit. Using a balance scale with no weights, what is the minimum number of weighings needed to guarantee finding it?
1
2
3
4
3. You have an unmarked 3-liter jug and an unmarked 5-liter jug, with unlimited water. Which sequence leaves exactly 4 liters in the 5-liter jug?
Fill the 5-liter jug and pour into the 3-liter jug; empty the 3-liter jug, transfer the remaining 2 liters into it, refill the 5-liter jug, then pour from it until the 3-liter jug is full.
Fill both jugs, empty the 3-liter jug, and pour from the 5-liter jug until the smaller jug is full.
Fill the 5-liter jug, pour into the 3-liter jug, and immediately refill the 5-liter jug.
Fill the 3-liter jug, pour it into the 5-liter jug, then fill the 3-liter jug again.
4. A square house has four exterior walls, and every wall faces south. A bear walks past the house. What color is the bear?
Brown
Black
White
Its color cannot be determined
5. Four people must cross a bridge at night using one torch. At most two cross at once, and a pair moves at the slower person's pace. Their crossing times are 1, 2, 7, and 10 minutes. What is the shortest possible total time?
19 minutes
20 minutes
17 minutes
21 minutes
6. Two ropes each take exactly 60 minutes to burn, but they burn at uneven rates. How can you measure exactly 45 minutes?
Light one rope at one end, wait until half appears burned, and then light the second rope.
Light both ropes at one end and extinguish one when the other finishes.
Light one rope at both ends, then light the second rope after 15 estimated minutes.
Light the first rope at both ends and the second at one end; when the first finishes, light the second rope's other end.
7. One hundred closed doors are numbered 1-100. On pass 1, every door is toggled. On pass 2, every second door is toggled, and this continues through pass 100. How many doors remain open?
9
50
10
100
8. Two fathers and two sons go fishing. They catch three fish, and each person takes home exactly one fish. What is the smallest possible number of people in the group?
Four people
Six people: three separate father-son pairs
Five people
Three people: a grandfather, his son, and his grandson
9. Four houses stand in a row, numbered 1 through 4. Noor lives in house 1. Jade lives immediately to the left of Luis. Mina does not live at either end. Who must live in house 4?
Jade
Luis
Mina
Noor
10. You face two doors: one leads out and one leads into a trap. One guard always tells the truth, and the other always lies. You may ask either guard one question. What should you do?
Ask which door the other guard would identify as the exit, then choose the opposite door.
Ask which door leads out and choose the door indicated.
Ask whether the left door is safe and choose it only if the answer is yes.
Ask whether the guard is truthful, then trust the answer.
11. Three switches are outside a closed room. Exactly one controls an incandescent bulb inside. You may operate the switches however you like, but enter the room only once. Which method identifies the correct switch?
Turn each switch on for the same amount of time, turn them all off, and enter.
Leave the first switch on for several minutes, turn it off, turn the second on, and enter; use the bulb's light and warmth to distinguish all three.
Turn the first and second switches on together, enter, and check whether the bulb is lit.
Keep all three switches off, enter, and touch the bulb to determine which switch controls it.
12. Each term in this sequence describes the digits in the term before it: 1, 11, 21, 1211, 111221. What comes next?
312211
211221
311221
1113213211
13. A clock takes 5 seconds to strike 6 o'clock, measuring from the first strike to the last. At the same steady rate, how long does it take to strike 12 o'clock?
11 seconds
12 seconds
6 seconds
10 seconds
14. Three boxes are labeled Apples, Oranges, and Mixed. Every label is wrong. You may draw one fruit from one box without looking inside. From which labeled box should you draw to determine all three contents?
The box labeled Apples
The box labeled Mixed
The box labeled Oranges
Any box works equally well
15. A father is four times as old as his son. In 20 years, he will be twice as old as his son. How old are they now?
The father is 32 and the son is 8.
The father is 48 and the son is 12.
The father is 40 and the son is 10.
The father is 36 and the son is 9.
16. What has cities but no houses, forests but no trees, and rivers but no flowing water?
A history book
A model railway
A weather report
A map