Find the sum of 3/7 and 1/21

Question

I need to find the LCM of this problem

Answer 1

Hi Portia--

 

when you have 3/7 and you have 1/21, you can actually divide the 7ths into 21sts also.

 

Draw a box divided into 7 sections (like a window with bars on it).

 

Shade in 3 of those 7ths.

 

Now, divide the whole box into 3 horizontal sections too, so it looks like a bunch of little squares in a box. If you count the little boxes, there should be 21 of them--because 3 x 7 is 21.

 

Now, see the shaded 3/7 you did before? Count the number of little squares there, and it should be 9/21.

 

Which you could add to your 1/21, and which proves that your LCM--the smallest multiple they both have--is 21.

 

(21 is the smallest multiple of itself, and it is also a multiple of 7).

 

Cool eh? (Well, at least to a 5th grade TEACHER, maybe not to a 5th grader!)

 

Good luck :-)

 

Matt in New York