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