Addition and Subtraction of Common Fractions

When you eat two-thirds of a pizza and your friend eats three-quarters (three-fourths) of the same size pizza, what is the total pizza you two have eaten? To calculate that, we need to first determine the LCD (lowest common denominator) of the thirds and the fourths. The LCD of that is 12. Thus, use equivalent fractions and change the two-thirds into eight twelfths by multiplying both the numerator and the denominator with 4. Do the same with the three-fourths. You still need to work with equivalent fractions, so multiply both the numerator of 3 and the denominator of 4 with a 3 so that the denominator is 12. Thus, you will have nine twelfths. In total, you will now have 17 twelfths. Now, convert this improper fraction to a mixed number of 1 and 5 twelfths. To determine the fraction of pizza left, you need to subtract the 1 and 5 twelfths from the original 2 pizzas. Do not forget that you can change the 2 pizzas into 1 and 12 twelfths pizzas. Having that, you can now easily subtract 1 and 5 twelfths from 1 and 12 twelfths to get an answer of 7 twelfths. Please remember, when adding or subtracting fractions, the denominators must always be the same.