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

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

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.