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Answer to Q19

My brother lives at the reverse of my house number. The difference between our house numbers ends in two. My house number is the lesser number. What is my brother's house number?

We are given two clues here, first the numbers must be the inverse of each other. That is if one is xy the other must be yx.

Second, subtracting the larger from the smaller the difference must end in 2.

This riddle can be solved through guess work and understanding how basic subtraction works.

Following the second clue, some might be tempted to find numbers with difference of 2 e.g. 6 and 4! With these you are saying the numbers are 64 and 46 but 64 - 46 = 18 (that ends with 8 and not 2)

But if you understand how basic subtraction works, you will know that when you start subtraction from left to right and the numerator is less that the denominator, you have to borrow 1 over from the next number! For instance 63 - 75, starting from the left 3 is less than 5 so you borrow 1 from 6 to make the 3 equals 13.

So to solve our riddle what is the lowest number we can make left-most high number (hope that is not confusing?!). That will be 0. Then borrowing one from the next number will make the 0 equals 10 so what can we subtract from 10 to make 2? That's 8! So we have 0 and 8

So is, 80 and 08 our number? Yes 80 - 08 = 72

Next clue says my house number is the lesser number. So my house number is 08

Question: What is my brother's house number?

Answer: 80

[Thanks to @sunkoitis, answer amended]

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387 viewsedited  Aug 8, 2019 at 09:50

Answer to Q19

My brother lives at the reverse of my house number. The difference between our house numbers ends in two. My house number is the lesser number. What is my brother's house number?

We are given two clues here, first the numbers must be the inverse of each other. That is if one is xy the other must be yx.

Second, subtracting the larger from the smaller the difference must end in 2.

This riddle can be solved through guess work and understanding how basic subtraction works.

Following the second clue, some might be tempted to find numbers with difference of 2 e.g. 6 and 4! With these you are saying the numbers are 64 and 46 but 64 - 46 = 18 (that ends with 8 and not 2)

But if you understand how basic subtraction works, you will know that when you start subtraction from left to right and the numerator is less that the denominator, you have to borrow 1 over from the next number! For instance 63 - 75, starting from the left 3 is less than 5 so you borrow 1 from 6 to make the 3 equals 13.

So to solve our riddle what is the lowest number we can make left-most high number (hope that is not confusing?!). That will be 0. Then borrowing one from the next number will make the 0 equals 10 so what can we subtract from 10 to make 2? That's 8! So we have 0 and 8

So is, 80 and 08 our number? Yes 80 - 08 = 72

Next clue says my house number is the lesser number. So my house number is 08

Question: What is my brother's house number?

Answer: 80

[Thanks to @sunkoitis, answer amended]

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