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# what is the common denominators for 1/3 and 2/5

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### 2 Answers by Expert Tutors

FREDERICK S. | Doc Fred, Super Tutor Kitsap - my specialties are: math, SAT, ACT...Doc Fred, Super Tutor Kitsap - my specia...
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Gracie,

While John's answer is okay, you may not have known abut prime numbers -

this is a middle school question... you only need to know:

Least Common Denominator = least common multiple (LCM) of the 2 denominators;

the least multiple of 3 and 5 is 3*5 = 15  and if you want to add these fractions do this

(5/5)*(1/3 ) + (3/3)*(2/5)  = 5/15 + 6/15 = (5 + 6)/15 = 11/15.

All done,  this time, but as it was a question this month,

if you have any more, just ask - I live nearby in your state. /Doc Fred

John R. | John R: Math, Science, and History TeacherJohn R: Math, Science, and History Teach...
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In order to find the common denominator of two fractions, you need to find the prime factors (prime numbers that are multiplied together to form the number) of each denominator.

A prime number can only be divided by 1 and itself.

Looking at the first fraction, the denominator is 3.  Since 3 can only be evenly divided by 3 and 1 (it is prime), its prime factor is 3 (1 is not considered a prime factor of other numbers).

In the second fraction, the denominator is 5.  Since 5 can only be evenly divided by 5 and 1 (it is also prime), its prime factor is 5.

Once the prime factors are found, they need to be listed.  In this case, the factors of the common denominator are 3 and 5.  (If a number appears in both denominators the, only list it once.)

Now multiply the the factors to get the common denominator (3X5=15)

If you are going to add these fractions, you must remember to multiply your numerators by the same number that you multiply your denominator.