by StaceyKoprince Thu Mar 06, 2008 1:33 am
Generally, if you're going to do this, you will have to estimate at least twice within the problem. In this case, choose with the idea that you want to minimize the error that you introduce via estimating - so if you choose the higher option on one fraction, choose the lower option on the other fraction.
For the examples you list, generally try to adjust the larger number because that will introduce less error. In both of these examples, that means adjusting the denominator.
We could make 5/18 either 5/15 = 1/3 or 5/20 = 1/4. I don't have to go as far to change it to 18 to 20, so I should choose that route.
For 6/20, I could do 6/18 or 6/24. 6/18 is closer, so go that route. (Note: although changing 20 to 18 is a difference of two digits, and changing 6 to 5 is a difference of only one digit, the proportional difference of changing the smaller number is much higher. A 1-digit difference from 6 is 1/6 or about a 17% difference, while the 2-digit difference from 20 is 2/20 or only a 10% difference.)
Stacey Koprince
Instructor
Director, Content & Curriculum
ManhattanPrep