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There are 100 new built houses in a row. Your job is to put the numbers on the front doors.
How many number 9's will you need?
1 as if the other houses don't use individual numbers to number their houses then there is only 1 number 9 house 
SarahAnne9 Correct answer! The common mistake is forgetting to include numbers 90 to 98
9,19, 29, 39, 49, 59, 69, 79, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99
Well done Sarah you are the first to get it right
saintsmoney That is INCORRECT it requires two nines for 99 so the answer there should be twenty. The COMMON MISTAKE is to count the house with a 9 in their number forgetting that 99 requires two.
The question asks how many NINES would you need - NOT how many houses have a NINE in their number.
SarahAnne9 Perhaps I misread it but saw "nineteen" somewhere, is someone else cheating?
It will depend on which side of the street they're on... the even side (on the right if walking away from the town) will take far fewer number 9s. .
11 as I am assuming they are numbered 1 to 100 all in a row. (Although you could say 22 if you used them upside down for the number 6 as well!).
Lynibis - 100 houses IN A ROW would be numbered 1 to 199 or 2 to 200
Even if counting the number nines up to 100 (on one sided numbering which is not usual) there are only 20 (not 22)
9 19 29 39 49 59 69 79 89 90 91 92 93 94 95 96 97 98 and two for 99
If using them in place of sixes (upside down) there'd be an additional 20:
6 16 26 36 46 56 60 61 62 63 64 65 66 (2) 67 68 69 76 86 96
So there are a few actual answers (if sixes were counted as nines) 40
If sixes were not counted as nines 20
If all nines were counted as sixes 00
They are numbered 1 to 100 in a row yes. My first answer was also 11 because the tendency is to get to number 89 and jump straight to 99 - completely missing numbers 90 through to 98. It's because you start counting 9, 19, 29 and so on, when you get to 89 you fall into the pattern trap and skip straight to 99 
Upside down 6's
davidstockport Great effort! 6's are not counted as 9's so you have the correct answer in there. Looks like you would be good at lateral thinking questions David so I will provide one in chat for tomorrow.
saintsmoney But you still got it wrong you didn't allow for two nines being required to make the number 99 
saintsmoney that's exactly what I did! Felt a fool when I realised I had missed all the 90s lol.
saintsmoney I look forward to it - I'll argue if I think you've got it wrong though,
Lynibis Don't worry about the maths Lyn - I'm always here to help - I found an easy way to count up to twenty 
Lynibis don't worry most people fall into this trap. It's not an intelligence test, you can be very bright and still blurt out 11 - thinking that the challenging part is remembering the two 9's for house number 99.
I am always honest when posing this riddle, by admitting that I got it wrong first time too.
saintsmoney And then you got it wrong a second time by saying 19 was the correct answer
We now know that SarahAnne9 who edited her answer is a cheat so must not be trusted with money
Look forward to similar brain teasers.
Lynibis I've posted a conundrum where a knowledge of history might help - so you're in with a good chance of getting the answer correct.
davidstockport Oi where does it show that I edited my answer lol I totally refute that allegation 
You wouldn't need any number 9s - you could use number sixes upside down.
The sixes and nines usually used for house numbering are identical.
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