The three digit number X Y Z is such that X , Y and Z are different primes and the number is divisible by each of them. What is the number?
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Short Python Solution. Directly yield output : 735
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There are 4 primes to use for XYZ
2357
The number has to be divisible by 3 of them, which means it has them as factors
210=2 3 5*7
210/2 = 105 as base? maybe
210/3 = 70 as base? no, zero rules it out
210/5 = 42 as base? maybe
210/7 = 30 as base? no zero rules it out.
Take the numbers 2 & 5...
multiples of 5 end in either 5 or 0, only 5 is prime
multiples of 2 can be 2 4 6 8 and 0, only 2 is prime
This means that it has to end with either 2 or 5, but with 210*N added on to it.
42? no
252? no
462? no
672? no
882? no
1092? no
105? no
315? no
525? no
735? yes! <---
945? no
There might be a neat way of doing this but I am going to go for a longer, more accessible method called casework.It is important to note that the primes available are 2,3,5 and 7. Now lets start by looking at the last digit Z. If Z=2, then we have 6 possibilities: (X,Y) = (3,5) , (3,7) , (5,7) + reverse order. However, if X or Y equals 3 then the digit sum must be divisible by 3 (divisibility by 3 rule) so (3,5) & (5,3) aren't valid. Also, X or Y can't equal 5 otherwise Z= 5, which contradicts Z=2. This excludes all possibilities for Z=2.
For Z=5, we have (X,Y) = (2,3) , (2,7) , (3,7) +reverse. Now using the divisibility by 3 rule (3,7) & (7,3) are the only possibilities left. Testing both gives XYZ = 7 3 5 = 3 × 5 × 7 2 . Now we just need to verify that there are no more solutions.
For Z=3: we use the divisibility by 3 rule again to exclude (2,5) & (5,2). Now if 5 is a digit then 5| XYZ but that implies that Z=5 so (5,7) &(7,5) are also excluded. (2,7) and (7,2) are excluded as well as XYZ is odd. So no solutions for Z = 3.
For Z=7: (2,5), (3,5) + reverse are excluded as 5 doesn't divide XYZ unless Z=5. Similarly (2,3) +reverse is excluded as 2 can't divide an odd number.
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Since 0 < X , Y , Z < 1 0 and all are prime we have X , Y , Z ∈ { 2 , 3 , 5 , 7 }
If any two are 2 and 5 the last digit of X Y Z is 0 which is not possible value.
We have ( X , Y , Z ) is a permutation of 2 , 3 , 7 or 3 , 5 , 7 , by divisibility rules for 2 and 5 each of these cases has Z = 2 and Z = 5 respectively.
All permutations are already divisible by 3 becasue 2 + 3 + 7 = 1 2 and 3 + 5 + 7 = 1 5 are both mutliples of 3
Divisibility rule for 7 says subtract 2 times the last digit from the others
For Z = 2 we have 3 7 − 4 = 3 3 and 7 3 − 4 = 6 9 , neither are divisble by 7
For Z = 5 we have 3 7 − 1 0 = 2 7 and 7 3 − 1 0 = 6 3 and 7 ∣ 6 3
So we have X Y = 7 3 and subsequently X Y Z = 7 3 5