You can always add fractions by putting the sum of "outer" and "inner" products over the product of denominators ("last").
Example:
.. 2/3 + 5/7 = (2*7 (outer) + 3*5 (inner))/(3*7 (last))
.. = (14 +15)/21
.. = 29/21 . . . . . an improper fraction, so can be written as a mixed number
.. = 1 8/21
In some cases, you will have to reduce the final fraction by dividing the numerator and denominator by the same number.
Example:
.. 2/8 + 2/6 = (2*6 +8*2)/(8*6) = 28/48
.. = (4*7)/(4*12)
.. = 7/12