Apple 100 Coins Blindfolded Puzzle Answer [Solved]

The 100 coins blindfolded puzzle answer: take any 10 coins from the table, put them in a small pile, and flip every coin in that small pile. Both piles will then have the same number of heads up. You do not need to know which coins are heads up, and you do not need to find a single head by touch. The flip does the work for you.

The 100 coins blindfolded puzzle, restated

Picture this scene. You sit at a table with 100 coins in front of you. Exactly 10 coins are heads up, and the other 90 coins are tails up. You wear a blindfold, so you cannot see any coin. You also cannot tell heads from tails by touch. Your task is to split all 100 coins into two piles so that each pile has the same number of heads up.

At first, the task looks impossible. If you cannot see or feel a head, how can you count heads into piles? Most candidates try to solve the puzzle by sorting coins, and that path leads nowhere. The real solution does not sort anything. Instead, it uses one simple fact: the number 10 in the puzzle is also the number of heads, and that match is the whole trick.

100 coins blindfolded puzzle: the two-step solution

Follow these two steps, in order, and the piles will always match.

Step 1: Count out any 10 coins. Slide any 10 coins to one side. Do not try to pick heads on purpose, because you cannot. Just count to 10 and stop. You now have a small pile of 10 coins and a large pile of 90 coins.

Step 2: Flip every coin in the small pile. Turn each of the 10 coins over, one by one. Leave the large pile untouched. That is the complete method. Two steps, no guessing, and no extra moves.

Notice what you did not do. You did not weigh the coins, you did not ask for help, and you did not remove the blindfold. You also did not need luck. Any 10 coins will work, every single time, and the proof below shows why.

A worked example with 3 heads in the small pile

Suppose the 10 coins you counted out happen to include 3 heads and 7 tails. You cannot know this while blindfolded, but let us follow the numbers to see what happens.

Before the flip, the small pile has 3 heads. Since the table started with 10 heads in total, the large pile must have the other 7 heads. So before the flip, the piles are uneven: 3 heads against 7 heads.

Now flip the small pile. Its 3 heads become tails, and its 7 tails become heads. After the flip, the small pile has 7 heads. The large pile still has 7 heads. The piles now match, exactly.

Stage	Small pile heads	Large pile heads
Before the flip	3	7
After flipping the small pile	7	7

Try another case to test the method. If your small pile starts with 0 heads, the large pile has all 10 heads. Flip the small pile, and its 10 tails become 10 heads, so both piles have 10 heads. If your small pile starts with all 10 heads, the large pile has 0 heads. Flip the small pile, and its 10 heads become tails, so both piles have 0 heads. The method holds at both extremes, and it holds for every number in between.

Why the 100 coins blindfolded puzzle always works

Call the number of heads in your small pile h. That number can be anything from 0 to 10, and you never need to know its value. Since the total number of heads is 10, the large pile must contain 10 minus h heads.

Your small pile has 10 coins in total, so if h of them are heads, the rest must be tails. That gives 10 minus h tails in the small pile. When you flip the small pile, every tail becomes a head, and every head becomes a tail. So after the flip, the small pile has 10 minus h heads.

That is the same number as the large pile: 10 minus h. The two piles match because the flip turns the small pile’s tail count into its head count, and that tail count was always equal to the large pile’s head count. This is a clean piece of algebra, not a lucky guess, and it is the reason interviewers love this puzzle.

The key insight is a change of view. Most candidates ask how to find heads they cannot see. The winning candidate asks how to use a number they already know. You know the total head count is 10, so you take 10 coins and flip them, and the unknown number cancels itself out.

The trap most candidates fall into

The most common wrong answer is to split the coins into two piles of 50 and hope the heads divide evenly. That plan fails because a random split gives no guarantee of equal heads. With 10 heads scattered at random, one pile of 50 could hold 2 heads while the other holds 8, and you would have no way to fix it.

A second trap is to flip coins at random and hope for balance. Random flips change the head count, but they do not create a rule that forces the piles to match. The correct method is exact, not random, and it works because the small pile size equals the total head count.

A third trap is to ask for a second blindfold removal, a scale, or a helper who can see. Those requests miss the point of the puzzle. The interviewer gave you everything you need: the total head count, the freedom to move coins, and the freedom to flip them. A strong answer uses only those tools.

What the interviewer is testing in the 100 coins blindfolded puzzle

This puzzle tests four skills at once. First, it tests whether you can work with an unknown quantity instead of trying to measure it. You never learn h, yet you still solve the problem. Second, it tests whether you spot the link between two numbers in the question: 10 heads, and a pile of 10 coins.

Third, it tests clear communication. A candidate who states the method, then proves it with one example and one line of algebra, sounds far stronger than a candidate who waves at the answer. Fourth, it tests calm under a strange setup. The blindfold is there to push you toward panic, and the best response is to slow down and count.

State the method first in your answer, then give the example, then give the proof. If the interviewer asks a follow-up, be ready for it. The natural follow-up changes the numbers: what if 7 coins were heads up instead of 10? The answer follows the same rule. Take any 7 coins and flip them, and both piles will match. The method generalizes to any head count, as long as the small pile size equals that count.

Frequently asked questions

Does the small pile need exactly 10 coins?

Yes. The method only works because the small pile size matches the total number of heads. If you take 9 or 11 coins, the algebra breaks, and the piles will not match in general.

Can you flip the large pile instead?

No, flipping the large pile does not give the same result. The proof depends on flipping the pile whose size equals the head count, so flip the small pile of 10 and leave the large pile alone.

Is this puzzle really asked at Apple?

Candidates have reported this 100 coins question in Apple interview reports, with the blindfold and the equal-heads split exactly as stated here. Reports are candidate-shared, so treat them as evidence of the question style, not as a promise for any single interview round.

What if you do not know the total number of heads?

Then the method cannot start, because you need that number to choose the small pile size. In the stated puzzle, the interviewer tells you the total is 10, and that fact is part of the question.

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