Compare how you would model and record finding the sum and difference of two rocks weighing 2/8 pound and 3/8 pound

Rule 1: In adding and subtracting fractions, make sure the fractions have the same denominator.
Rule 2: In the event that the denominators are not the same, find the least common multiple of each denominator and multiply each fraction by a number that will produce the least common multiple.
Rule 3: Perform the operation and simplify the resulting fraction.

2/8 pounds and 3/8 pounds
*since both fractions already have common denominators we proceed to performing the operation on the numerators.
*add or subtract numerators and place on top of the common denominator. 
*simplify the resulting fraction.

Adding fractions:
2/8 + 3/8 = (2 + 3) / 8 = 5/8

Subtracting fractions:
2/8 - 3/8 = (2 - 3) / 8 = - 1/8