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 Monty Hall Simulation
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The key to this math riddle is realizing that the one place must be zero. 888 + 88 + 8 + 8 + 8 = 1,000
One of the 'fathers' is also a grandfather. Therefore the other father is both a son and a father to the grandson.
In other words, the one father is both a son and a father.
The most common digit is '1.' Can you figure out why? No hints until you try the next riddle because the next riddle is closely tied to this one.
Part II. What digit is the least frequent between the numbers 1 and 1,000?
0 is the least common digit even though 1,000 has three zero's !
Explanations for both riddles
The digits 0 through 9 all follow the same pattern there is exactly 1 occurrence of each digit for every ten numbers.
 For instance the digit 2 appears once between 10 and 19, at 12. And 2 appears once between, 30 and 39 at 32.
 However, each of the digits 1 through 9 also appear in other numbers in the tens and hundreds place
Again, let's look at 2 which appears in 20,21,22, 23, etc.. as well as 200,201, 202,203..
So to figure out how to answer the first riddle you had to see what distinguishes the number 1? Only that we are including 1,000 which would be the first '1' in a new series of ten! In other words, the digit 1 only has a single extra occurrence (301 occurrences) compared to 2 or 3 or 9 which each have exactly 300 occurrences.
The reason that zero has the least (BY FAR at only 192 occurrences) is because zero does not have any equivalents to 22, 33, 44, 222, 3333 etc..
There are many ways of explaining/thinking about this truly brain bending riddle! It all boils down to the fact that the lawyers's math is incorrect.
They did NOT spend $9 • 3 + $2.
They spent exactly $27 dollars. $25 for the room and $2 for the tip. Remember they got exactly $3, in total back.
Another way to think about the answer to this riddle is to just pretend that the bellhop refunded $3 to the lawyers (rather than giving them $5 and receiving $2 back).
If the lawyers get $3 back and each takes $1. They they spent exactly $27 dollars.
The answer is 4
11 cartons total
7 large boxes (7 * 8 = 56 boxes)
4 small boxes (4 10 = 40 boxes
11 total cartons and 96 boxes
The key to solving this riddle is realizing that you have to take the rabbit over first and the switch the fox with the rabbit. See step 2.
Step 1Take the rabbit to the other side
Shore  Other Side 
Carrots
Fox 
Rabbit 
Go back and get the Fox and switch it with the Rabbit
**The key here is that the carrots and the rabbit are not being left alone.
Shore  Other Side 
Carrots
Rabbit (Not left alone) 
Fox 
Take the carrots across
Shore  Other Side 
Rabbit  Fox
Carrots 
Go back and get the rabbit
Shore  Other Side 
Rabbit
Fox Carrots 
Every farmer's part is 1/3(45+75) = 40 sacks.
Charlie paid $1400 for 40 sacks, then 1 sack costs $1400/40 = $35/sack.
Adam got $35*(4540)=35*5 = $175.
Ben got $35*(7540)=35*35 = $1225.
Answer: Ben $1225, Adam $175
Why would he need to go back to get the last hint after seeing the number on the house next door?
Because the sum of their ages ( the number on the house) is ambiguous and could refer to more than 1 trio of factors.
{ 2, 2, 9}
If you list out the trio of factors that multiply to 36 and their sums, you get :
 1 1 36 = 38
 1 2 18 = 21
 1 3 12 = 16
 1 4 9 = 14
 6 6 1 = 13
 2 2 9 = 13
 2 3 6 = 11
 3 3 4 = 10
Since the number on the house next door is not enough information there must be more than 1 factor trio that sums up to it, leaving two possibilities: { 6, 6, 1} , {2, 2, 9} . When she says her 'oldest' you know it can not be {6,6,1} since she would have two 'older' sons not an 'oldest'.
There actually is a mathematically correct answer to this riddle: You should indeed change your choice. If you don't believe me, just try out our free online Monty hall simulation.
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Look at how many closed areas there are.
 9999 has 4 closed areas (the top of the '9')
 8888 has 8 closed areas, the top and bottom parts of the 8 and there are no other digits
 1816 has 3 closed areas, (top and bottom of 8 and bottom of 6, and it has 2 other digits ( 3*2=6)
 1212 has 0 closed areas,(0*4=0)