Hannah buys ⅔ pound of roast beef. She uses ¼ pound to make a sandwich for lunch. How much roast beef does she have left?

5/12

Explanation:
With unlike denominators find the Least Common Denominator (LCD)

LCD = 12
3x4=12

Multiplying numerators and denominators to get the LCD in all fraction denominators
=2×4/3×4 − 1×3/4×3

Then rewriting the equation with the equivalent fractions
=8/12 − 3/12

With like denominators we can operate on just the numerators
=(8−3)/12
=5/12