Be Miss Wormwood and teach Java to add fractions.

        a       c        a*d   +   b*c
      ---- +  -----  =  ----------------
        b       d             bd


        top = a*d + b*c

        bottom = bd


 BigFraction add(BigFraction that)

a: num
b: denom
c: that.num
d: that.denom


        top = num.multply(that.denom).add(denom.multlpy(that.num))
        bottom = denom.multiply(that.denom)