13/30, because you find the common denominator, 30.
From there, what you do is think, what times 5 is 30? (Talking about 3/5) 6! So you multiply 3 and 6, and get 18/30.
Next, do the same with 1/6, and you get 5/30.
Finally, subtract 5/30 from 18/30, and you get 13/30.
Hope this helps!
