Hannah buys 2/3 pound of roast beef. She uses 1/4 pound to make a sandwich for lunch. How much roast does she have left?

What we know: Hannah buys 2/3 LB

Uses 1/4 LB

Well, the word "left" tells you that you would have to subtract 1/4 from 2/3. This is when you will have to find out the Least Common Denominator for both fractions.

Now, just subtract:

2/3-1/4

And you will end up with an answer of 5/12.

Take LCD of 3 and 4 = 12

(2 * 4) / 12 - (1*3) / 12

8/12 - 3/12

= 5/12 pound of roast beef is left.