Five Brilliant Brain Teasers For The Week (21/07/25)

Settle in for another round of riddles and reasoning. This week's set includes two easy warm-ups, two moderate thinkers, and one tough cookie to round things off. Grab a notepad or a friend and enjoy the mental stretch.

Scroll down slowly — answers are at the end!

🧠 1. The Fast Fingers (Easy)

You’re in a race.

You pass the person in second place.

What place are you in now?

🧠 2. The Coin Toss (Easy)

You have a fair coin.

You flip it three times.

What is the probability it comes up heads at least once?

🧩 3. The Suspicious Sack (Medium)

You’re shown five sacks of gold coins. Each should contain coins that weigh exactly 10 grams each.

However, one sack is full of counterfeit coins weighing 9 grams each. You have a digital scale but can only use it once.

How do you figure out which sack contains the fake coins?

🧩 4. The Four Digit Lock (Medium)

A briefcase is locked with a four-digit code.

You’re given three incorrect guesses:

682: One digit is correct and in the correct place

614: One digit is correct but in the wrong place

206: Two digits are correct but both in the wrong places

What is the code?

🧠💥 5. The Island of Green-Eyed People (Hard)

On an island, there are 100 people, each with green eyes — but they don’t know their own eye color. They are not allowed to speak about eye color or look into reflective surfaces.

However, they can see everyone else's eyes. One day, a visitor says aloud:

“At least one of you has green eyes.”

What happens next?

✅ Answers Below — Ready?

1. The Fast Fingers

If you pass the person in second, you take their place.

Answer: You’re now in second place.

2. The Coin Toss

The only way you get no heads is if all flips are tails:

Probability of tails 3 times: ½ × ½ × ½ = 1/8

So, heads at least once: 1 – 1/8 = 7/8

Answer: 7⁄8

3. The Suspicious Sack

Label the sacks 1–5. Take:

1 coin from sack 1

2 coins from sack 2

3 coins from sack 3

4 coins from sack 4

5 coins from sack 5

Weigh all 15 coins at once.

If all coins were genuine: 15 × 10g = 150g

If one sack has 9g coins, the total will be less by X grams, where X = the number of coins from that sack.

So if total weight = 147g → the 3g loss = Sack 3 is fake, and so on.

Answer: Use differing numbers of coins to pinpoint the lighter sack by weight difference.

4. The Four Digit Lock

Let’s analyze:

682: One correct and in the right place

614: One correct but in the wrong place

206: Two correct, both in the wrong place

Solution: 042

Try plugging it into all clues — it fits:

In 682: 2 is correct and in the correct place (third digit)

In 614: 4 is correct but not in third → it’s second here

In 206: 0 and 2 are correct but both in the wrong places → fits!

Answer: 042

5. The Island of Green-Eyed People

Here’s what happens:

Each person sees 99 green-eyed people.

They all think: "If I don’t have green eyes, they’ll all leave on day 99."

But nobody leaves on day 99.

So on day 100, they all realize: “I must have green eyes too!”

All 100 leave the island on day 100.

Answer: All 100 leave the island on the 100th day.