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

First we need to make both fractions equal to each other, and that means having to have the denominators the same.

What both 3 and 12 easily go into is 12.

So we need to make both fractions have a denominator of 12, so we multiply.

2/3 * 4/4 = 8 / 12

1/4 * 3/3 = 3 / 12

Now, we know that she has 8 / 12 pounds, and she uses 3 / 12 of it, so now we subtract to find how much she has left.

8 - 3 = 5

Out of the 12 pounds of roast beef, she now has 5 / 12 left.

Well you have to find the common denominator. You have to look for the common multiples. When you find it (12) you have to multuply the numerator the same you multipied the denominator. So it's 8/12-3/12. 8-3=5 and the denominstor stays the same.