Which of the following is a prime number?

A prime number (or a prime) is a natural number or positive integer greater than 1 that has no positive divisors (natural numbers) other than 1 and itself. 212 is divisible by 2 (and other natural numbers such as 106), 485 is divisible by 5 (and other natural numbers such as 97) and 573 is divisible by 3 (and other natural numbers such as 191). The number 313 is only divisible by 1 and itself.

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60 Comments

ebitnet

Here's simple check you can do w/o a calculator. The toss up was between 313 and 573, the other two being even or divisible by 5. You can verify that 573 is divisible by 3 by simply adding 5 + 7 + 3 = 15, which is also divisible by 3, leaving only 313. Interestingly enough, 313 is also a prime pair with 311 and can be represented as 12^2 + 13^2. Another well known number with this property is 8675309 which in addition to being Jenny's phone number, is prime.

tkudera

I answered wrong,but I learned something,which is what quizzclub is all about!!

Ian Swindale

Having been totally hopeless at maths when at school I didn't even know what a prime number was!!

Mary Clinton

Me too! I love numbers now! RN too but retired!! Eleanor Flower,

myers nj

ebitnet, Where were you when I had this in High School and college. This makes sense😆

redrose

Eleanor Flower, My husband cannot add 1+1 and come up with two but at work if he gets things in fractions he can immediately say that's whatever it is in decimals.

Jim Adams

ebitnet, Good horse, but figuring which of the two was prime was easy enough.

lorriefiel

I am terrible at math but got the answer correct by dumb luck.

Eleanor Flower

Ian, I am with you! Hopeless when it comes to math, my spouse was a whizz! I Passed algebra with a 76! Was lucky to get out of the class! Hate numbers! Yet went into nursing and dealt with centimeters etc very well! How do you explain that?

Char

Got it so wrong..

Take A. Guess

ebitnet, Nice additional response !

docluci

Took a little while to check the math, never knew the short trick everyone mentions below!!!!!

Howard Groopman

Easy to rule out 212 and 485 because they're obviously divisible by 2 and 5, respectively. 573 was excluded because the sum of its digits is 15, and the trick to that is if the sum of the digits is divisible by 3, then so is the whole numeral. That left only 313.

Lisa Rodrigues

If the digits of a number combined create a one digit number which is divisible by 3, that number is divisible by 3. Numbers whose final digit is 0 or 5 are divisible by 5. Even numbers are divisible by 2. It was a simple process of elimination.