Hi all, need a simple fraction help

Hi guys, my name is sdfghnjhgfdcfvgbhnjhgfdsx. I was following a math site and there were certain methods they used that confused me. Like which is the right one to use. here's the examples:

7 + 2 3/4. 2 3/4 is a mixed fraction. so 4 x 2 + 3 = 11. making this

7/1 + 11/4 .. the LCD(Least Common Denominator) is 4. so they multiplied 7 and 1 by 4.

7 x 4 = 28 and 1 x 4 = 4... so

28/4 + 11/4 = 39/4 < i get it <-- This Method worked out Adding or Subtracting Fractions
Without Common Denominators

the next example...

Write the fraction as an equivalent fraction with the given denominator. 3/5 with the denominator of 20. so if 5 x 4 = 20 (as the denominator). i am told to multiple 3 x 4 = 12 to make the fraction equivalent. so the equivalent fraction of 3/5 with a denominator of 20 is 12/20. basically multiplying both 3 and 5 by 4. <-- This Method works out Equivalent Fractions

now here's my problem. the 2 examples are pretty similar, but the 1st one wasnt just to write an equivalent fraction, it also was the addition of 2 fractions. so i'm trying to use this method for addition or subraction of fractions. But now im given this task that involves adding 2 fractions once again. but this time, their solution is not this method i just showed, it's now the Equivalent Fraction method i showed which was simply finding out the equivalent fraction. It is now confusing me, here's the task:

3/4 + 2/5 - 7/10

so remember im using method 1. 20 is divisable by all 3 denominators. so like the example showed i multiply all fractions by 20. 3 x 20 = 60, 4 x 20 = 80 etc.. but now they are telling me this isnt right. That i should use the method they used with Equivalent Fractions. So 20 is already the denominator. lets take the 1st fraction (3/4), if 4 x 5 = 20(denominator.) we must also multiply 3 by 5. so we get 15/20. i get it, but why did they not use the method they explained for addition? why have they confused me by using this other method? how will i know which one to use when presented with tasks like these?

please help me(Crying)

The example you gave for addition had denominators of 1 and 4, so they multiplied the numerator and denominator of 7/1 both by 4 to get 28/4. This is no different than finding the least common multiple for 1 and 4. I think what's confuisng you is that the only difference between the two examples is that the first started with a mixed fraction - once it got converted to a "regular" fraction the technique is the same as in example 2. Bottom line - once you have the things you adding (or subtracting) as "regular" fractions, next step is to find the LCM for the denominators.

well im not sure. if we remove the mixed fraction and compare the 2:

7/1 + 11/4
and
3/4 + 2/5 - 7/10

the 1st one multiplied 7/1 both by 4 LCM. but in the other task what's confusing me is they didnt miltiply the fractions by the LCM 20 like they did in the 1st one. Instead they used 20 as the denominator then mulitplied the nemerator by a number that would result in 20. Why didnt they multiply both the numerators and denomerators by 20 like the previous?

Quote:

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Originally Posted by sdfghnjhgfdcfvgbhnjhgfdsx

well im not sure. if we remove the mixed fraction and compare the 2:

7/1 + 11/4
and
3/4 + 2/5 - 7/10

the 1st one multiplied 7/1 both by 4 LCM. but in the other task what's confusing me is they didnt miltiply the fractions by the LCM 20 like they did in the 1st one. Instead they used 20 as the denominator then mulitplied the nemerator by a number that would result in 20. Why didnt they multiply both the numerators and denomerators by 20 like the previous?

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Because if you multiplied top and bottom of each fraction by 20, you will end up with the three fractions having different denominators. You can only add or subtract fractions when they have the same denominator!

ok that computed instantly in my head. why was i so stupid not to think of that? thanks both of you for offering me with your help:)

You probably used up all of your brain power remembering your username when you logged on!

Quote:

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Originally Posted by hallsofivy

you probably used up all of your brain power remembering your username when you logged on!

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hehehe:)