Math Goodies is a free math help portal for students, teachers, and parents.
|
Interactive Math Goodies Software

testing left nav
Math Forums @ Math Goodies
Home | Profile | Active Topics | Members | Search | FAQ
 All Forums  Homework Help Forums  Basic Math and Pre-Algebra  Math grade 8 New Topic  Reply to Topic  Printer Friendly
Author  Topic

Katy
Junior Member

6 Posts

 Posted - 09/25/2009 :  19:44:38 A number magician tells you: Pick any postive integre and mutiply it by 18. Delete one of the non-zero digits of the poduct.Tell me the sum of the remaning digits of the product and I will tell you what digit you deleted.Explainthis trick.

Katy
Junior Member

6 Posts

 Posted - 09/25/2009 :  19:52:33 Any help is greatly appreciated this is kinda urgent

anonmeans
Senior Member

USA
37 Posts

 Posted - 09/27/2009 :  08:30:08 Pick a variable for the unknown number. Create expressions representing what you're doing to the number. Simplify. See what you get.If you get stuck, please reply with your steps and reasoning, starting with your variable and the expression you got after multiplying by 18.

Subhotosh Khan

USA
9117 Posts

 Posted - 09/28/2009 :  09:52:57 quote:Originally posted by KatyA number magician tells you: Pick any postive integre and mutiply it by 18. Delete one of the non-zero digits of the poduct.Tell me the sum of the remaning digits of the product and I will tell you what digit you deleted.Explainthis trick.Any multiple of 9 will have a sum of digit that is divisible 9.18 is divisible by 9.So when you are looking at multiple of 18 - it is a multiple of 9 too.Thus multiple of 18 will have "sum of digits" divisible by 9.suppose you chose 18 * 43 = 774and you decide to drop 4 - so sum of remainig digit is 14.You must have started from a sum-of-digit of 18 - so you must have dropped 4.Try other numbers say 18 * 69 = 1242You decide to drop '4' and give me the sum '5'You must have started from '9' and you have dropped '4' (= 9-5)
Topic
 New Topic  Reply to Topic  Printer Friendly Jump To: Select Forum New Visitor Forum       Testing Forum Homework Help Forums       Basic Math and Pre-Algebra       Algebra       Geometry and Trigonometry       Pre-Calculus and Calculus       Probability and Statistics       Standardized Test Preparation Help       Miscellaneous Math Topics Educator Forum       Teacher Talk Parent Forum       Parent's Place  -------------------- Home Active Topics Frequently Asked Questions Member Information Search Page
 Math Forums @ Math Goodies © 2000-2004 Snitz Communications