Drexel dragonThe Math ForumDonate to the Math Forum

Ask Dr. Math - Questions and Answers from our Archives
_____________________________________________
Associated Topics || Dr. Math Home || Search Dr. Math
_____________________________________________

Primes and Number Riddles


Date: 4/15/96 at 21:17:0
From: Anonymous
Subject: Pre Algebra: Finding primes as the result of solving number 
riddles

I am having difficulty understanding this riddle.  Please help me to 
find the proper method to solve this and future riddles like it.  
Thanks and here is the problem as it appears in its entirety.

Three different one-digit primes
Produce me, if you're using times;
If my digits you add,
Another prime will be had.
Two answers - and nothing else 
rhymes.

Thank you very much for your assistance!


Date: 4/22/96 at 20:19:12
From: Doctor Joshua
Subject: Re: Pre Algebra:  Finding primes as the result of solving 
number riddles

Hi!

By process of elimination, there are only four one-digit primes:
   2,3,5, and 7.  

Multiplying any three of these together:

2X3X5  =  30  Sum of digits = 3, this works!

2X3X7  =  42  Sum of digits = 6

2X5X7  =  70  Sum of digits = 7, this works as well.

3X5X7  =  105 Sum of digits = 6

So the answers to the riddle are 30 and 70.

-Doctor Joshua,  The Math Forum
 Check out our web site!  http://mathforum.org/dr.math/   
    
Associated Topics:
High School Puzzles

Search the Dr. Math Library:


Find items containing (put spaces between keywords):
 
Click only once for faster results:

[ Choose "whole words" when searching for a word like age.]

all keywords, in any order at least one, that exact phrase
parts of words whole words

Submit your own question to Dr. Math

[Privacy Policy] [Terms of Use]

_____________________________________
Math Forum Home || Math Library || Quick Reference || Math Forum Search
_____________________________________

Ask Dr. MathTM
© 1994-2013 The Math Forum
http://mathforum.org/dr.math/