[Blue_Ridge_Mtn_Geek]

in general,

1/N + 1/M = (N+M)/(NxM),

so this doesn't take much effort - - even a mental midget could do it without paper when the denominators are as small as 3 and 4!

[Dinsdale]

Even more generally.

X/N + Y/M = ((X*M)+(Y*N))/(N*M)

It's will be the correct answer but not necessarily the least common denominator.