# A math riddle for you. Feel free to submit your own.

If your friend’s son is four years old and his little brother is half his age (two years old), how old will the younger brother be if the older brother is 100?’

98. The “half his age” is a red herring and useless information. Many people will insist the younger brother is 50.

What is 2+2? Is it 5?

No, it’s 4. Mention of 5 was a red herring :rolleyes:

You’d be surprised at how many people are fooled by the first riddle. I’ve used it as an ice breaker at conferences for years.

Divide 30 by a half and add 10

too many people don’t think it through

(Not Dopers of course

{70 not 25}

I can easily see that riddle working.

People engaged in idle conversation often say the first and most obvious answer.

What is it a conference for people with severe learning disabilities?

That’s not a math riddle!

There are people who could envision two young siblings growing up, and one ending up 50 years older than the other? My mind boggles.

A better riddle of this type, of which there are countless examples, is:

John is 3 times as old as Billy. In 6 years, John will be twice as old as Billy. How old are they now?

This is simple enough for most people to figure out (although probably not the ones who answered the OP’s question incorrectly).

Pretty much my reaction. I thought, is there some word trickery I’m missing here? I hesitated for a minute, went with my answer, expected to have an “oh, shit! how did I miss that?” moment, but then, no, it was exactly the answer I’d expect anyone would get. I mean, hopefully the OP at least leaves off the parenthetical “two years old” part. Then I could see at least some people getting tricked.

Not really. That’s what makes riddles such as that one fun for some people. They quickly figure (especially if asked orally) they just cut the age in half. After they’re told the answer, they slap their forehead because they fell for it. Here’s a non-math riddle told orally following that premise that is told by kids:

Spell so (kid spells so)
Spell foe (kid spells foe)
Spell no (kid spells no)

What do you do at a red light?

Of course kids know that you stop. But if they fall for it…

As a kid, one of my favorite misdirection groaner versions of this type of puzzle started with “Ok, you’re [or I’m] driving a bus. At the first stop X amount of people get on. At the next stop, half the people get off, and 3 people get on.” And then you just spin in with some types of seemingly complicated, but doable math for a couple rounds. At the end, the question is “what is the name of the bus driver?”

Well, it’s there in the first sentence.

My father drove me nuts, because I’ve heard my father tell that riddle, and completely forget the first sentence as, you know, kinda being the key to it all, so it just ended up completely non-sensical in the end. Or maybe he was trying to some abstract Kaufmann shit, I don’t know.

It would probably have worked better if you left out the parenthetical “(two years old).”

[ol]
[li]Three horses are running at 30 miles per hour. What is the speed of one horse?[/li][li]A bucket of 90 degree water is added to a bucket of 90 degree water. How hot is the resulting water?[/li][li]You have a dozen cookies. You eat all but three of them. How many cookies do you have left?[/li][/ol]

A man offers you a car if you can choose which one of three doors it’s behind. Behind the other 2 are goats. After you choose, he opens one of the other doors and reveals a goat. He then offers you the chance to switch doors. Should you, or does it not matter?

You should definitely not switch. The only reason someone would add this option in after the fact is if you chose the car straight away.

Some of my favorites:

Joey has two pencils. One is an expensive pencil, and one is a cheap one. The expensive pencil costs \$1.00 more than the cheap one. He spent \$1.10 total. How much was the cheap pencil?

The others I’ll post links to:

I do leave it out when speaking the riddle. I’m not sure why I added it in written form.

A man drives to visit a friend. He averages 20 miles per hour for the trip. How fast does he have to drive on the trip back home to average 40 mph for the whole round trip?

No, it’s not 80, or whatever number you were thinking of. It’s impossible. He’d have to make the return trip in zero time to average 40 mph for the round trip.

nm

I was lazy and linked to the same riddle instead of writing it out. Definitely a good one as it’s not too easy or too hard.

The way I’ve always heard this one is the final question is “how many stops did the bus make”? I think this is better because the answer similarly turns out to be much simpler than expected, but in this case the questionee won’t be able to give the right answer (assuming he was keeping track of the passengers rather than the stops).