Question 979041
Not sure your question's meaning.


Addition is wanted:  {{{3/15+2/5}}};


You want to raise {{{2/5}}} to higher terms, meaning its equivalent with a larger denominator.  Which denominator?



Look at the two denominators which are present:


{{{system(15=3*5,5=5)}}}, and the factor of 1 is understood.
Lowest common denominator is {{{3*5=15}}}.
Raise the terms of {{{2/5}}} into the equivalent with denominator 15.
-
{{{3/15+(2/5)(3/3)}}}

{{{3/15+6/15}}}


{{{9/15}}}


{{{(3*3)/(3*5)}}}


{{{highlight(3/5)}}}