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JudyW JudyW is offline
Joined: Aug 1999
Posts: 82
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JudyW's Avatar
Joined: Aug 1999
Posts: 82
Founding Member
Old 01-17-2006, 06:26 PM
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Many years ago a more experienced teacher taught me this trick and I found that it worked great and we didn't end up with large denominators that had to be simplified.

Example: 3/4 x 4/12=
Children look at the larger of the two denominators (12). Ask yourself: Can I divide the smaller (4) into the larger (12) evenly. If yes, the the larger (12) is the LCM. They then changed 3/4 to 9/12 by multiplying the numerator and denominator by 3.

Example: 3/4 + 2/9=
Children look at larger denominator and ask themselves if the smaller can be divided into the larger evenly. In this case the answer is no. So we then multiply the larger numerator by 2 and get 18 in this case. Ask if 18 can be divide by 4 evenly. No. The next step is to multiply 9 by 3 to get 18. Can 18 be divided by 4 evenly. No. So we would go on to multiply 9 x 4 for 36. Can 36 be divided by 4 evenly? Yes, so 36 is the LCD and we would have to change both to an equivalent fraction.

My fifth graders caught on quickly. I introduced them to this after we had practice listing multiples to find the LCD several times for understanding.

As far cross multiplying, I only encouraged them to use it to determine if two fractions were equal or not on those higher level questions. Example: Is 2/3 and 4/10 equivalent. 2x10=20;3x4=12 so the answer would be no.

Hope some of this makes sense to you. Do you think the children have so much trouble with fractions because they rarely use them in their daily lives or is the concept introduced to them before they are developmentally ready? We've debated the issue at my school for years. It would be interesting to find out how others feel about it.
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