When adding fractions, and you have to find the lowest common denominator. How do you know when to change the numerator? I am really confused!!!!

Question

radar · Answer

What ever you do to the denominator you do to the numerator otherwise you change the value of the fraction.  If you multiply the denominator by 2 then multiply the numerator.$$1/3 + 3/4=$$  What is the smallest number that both 3 and 4 will divide into with out a remainder (LCM).  In this the LCM is 12 now examine the first fraction, its denominator is 3.  What must 3 be multiplied with to equal 12?  Your answer is 4.  So if I multiply  the denominator by 4 I MUST multiply the numerator by 4.  The first fraction becomes 4/12.  Treat next one in a similar manner.

anonymous · Answer

Okay, so the next fraction will be 3/12!!!!!

idily101 · Answer

yes

anonymous · Answer

Okay, so what if i had a question like 1/4 + 3/10 + 2/5.The LCD is 20 right

idily101 · Answer

yup

idily101 · Answer

1/4 would be 1/5 3/10 would be 6/20 and 2/5 would be 8/20

idily101 · Answer

do ypu understand how I got those fractions

anonymous · Answer

How would 3/10 be 6/20 and 3x6 is 18??? I dont understand!!!

radar · Answer

No the next fraction in y example was 3/4  we had already decided that 12 would be the LCD for those two fractions.  You then multiplied the denominator by 3 and you got your 12.......But you failed to multiply the numerator by  3 !!!!! Remember what you do to the denominator you MUST do to the numerator.  Multiplying it by 3 gives you 9/12 not what you got of 3/12  3/12 reduces to 1/4  not 3/4.  Look at 9/12 it will reduce to 3/4, see you did not change the value, only the appearance!

radar · Answer

Khiyah, take a breath and lets do your example. 1/4+ 3/10+2/5.  Your choice of 20 was very good, it is the LCM.  Convert your fractions with a denominator of 20 WITHOUT CHANGING THEIR VALUE.
1/4:  To get a value of 20 I must multiply denominator by 5, what must I multiply the numerator by 5.  1/4=5/20
3/10 a 2 is used to get 20 use the 2 for the numertator also. 3/10 = 6/20.  Do you get the procedure then you should get 8/20 for the last fraction.  Please note we did not change the value of the fraction.