a piece of limestone weighs 3/5 of a pound. A piece of slate weighs 4/7 of a pound. What is the difference in the weight of the two rocks?
You need to get a common denominator of
35. Multiply the top and bottom of 3/5 by 7 and multiply the top and bottom of 4/7 by 5. Then, keep the denominator and subtract the numerators to give you the difference in weights.