To convert a mixed number to an improper fraction, multiply the denominator by the whole number, add that total to the numerator and put the whole thing over the original denominator.
A mixed number.
I wouldconvert both to improper fraction,find a common multiple (CM),calculate equivalent fraction with CM as the denominator,add the numerators,change the improper fraction to a mixed fraction, if required,simplify the fractional part of the mixed fraction - if appropriate and required.
If you are using the same number, then the improper fraction and mixed number are equal.
Improper Fraction.
18/12 is an improper fraction. It's equivalent to the mixed number 1.5 .