Question

A scientist had 3/5 liter of solution. He used 1/6 of the solution for an experiment. How much solution did the scientist use for the experiment?

Answer 1

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!