It's Math Riddle Monday!

No really, it is. I'm not kidding.
Here is this Monday's question:
You're in a dark room with 50 quarters, 18 of which are heads up. You are allowed to move around the coins or flip some or all of them, if you wish. Problem is, it's too dark to tell what you're moving or flipping (no, you can't figure it out by touch either). Your job is to split the coins into two groups, each of which has the same number of heads up coins. How do you accomplish this?
Post your answers in the comments field. I will post the correct answer when I figure it out.

Note to George: This is the question I called you about on Saturday. Thanks for calling me back after "a few minutes."


Jennifer said...

The spendthrift comp-lit major's answer: spend all the quarters. 0 divided by two is 0.

Mike said...

It's math and you're a girl -- you are at a disadvantage. Actually it's logic and you're a girl -- you are at more of a disadvantage than earlier... :)

Where's math-guy Breedo? He'd have this thing figured out and then argue with himself that he is wrong -- and win!


Jennifer said...

I have no idea where Breedo is. I'm eagerly awaiting the solution to this math puzzle.

On a completely unrelated note have you seen this?

George said...

Here's a solution:

Separate out 18 quarters. Now you have two piles; let's call the number of heads in the group of 18 "n".

Since there were 18 heads total, the number of heads in the group of 32 is perforce 18 - n.

Now flip all the coins in the group of 18 over: instead of n heads in that group, you will now have 18 - n heads.

QEF. (which would make a very good WV)

This took almost five minutes to figure out, though I'm betting I'd heard it somewhere ten or more years ago.

Real WV: ogpafj <-- result of typing my email address too fast, wearing oven mitts

Mike said...


You are, in fact, correct. I couldn't take credit for solving this one as I had seen it before and solved it.