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 outAdding 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 outEquivalent 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