easy.... 59
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what number is prime to 40 29 - it's a prime number (divisible only by 1 and itself) and if you sum the two digits 2 + 9 = 11
9 is not prime
We note that '125' ends in '5'. So it will divide by '5', which is a prime number. Hence 5)125 5)25 5)5 = 1 Keep dividing by any prime number, that will go into the answer, in this case '5'. So 5 x 5 x 5 = 5^(3) = 125 NB Any number that ends in '5' or '0' will divide by '5'. Any number that ends with an EVEN number will divide by '2'. Any number were the digits sum to '9' will divide by '9'. e.g. 27 ; 2 + 7 = 9 27/9 = 3
The product of two prime numbers can never be another prime number, the numbers that you multiplied are factors of the product. (example, 9 times 5 is 45, 9 and 5 go into 45)
As even numbers, none of 138, 172 or 432 can be prime.As the sum of the digits of the number is equal to 9, 441 cannot be prime.