Your question is not completely clear. When you multiply fractions, you don't need to worry about similar or dissimilar. When you add or subtract fractions, you can make them similar this way. Multiply each by 1 written in terms of the denominator of the other.

__Example__2/5 + 1/3 = (2/5)*(3/3) + (1/3)*(5/5) (note that 3/3 = 1, written in terms of the denominator of the fraction 1/3, which is 3) = (2*3)/(5*3) + (1*5)/(3*5) = (6 + 5)/15 = 11/15