The denominator of a fraction is the bottom number. It tells us how many equal parts there are. When we add or subtract fractions, it is important that the denominators are the same for all of the fractions added or subtracted. This is called the common denominator. Common means the same. To do this we must find a common multiple (least common multiple) among the denominators. That means find a number that both denominators will "go into." For example: 1/5 and 1/3 has a LCM of 15 because 1/5 would have to be divided into thirds (because of the 1/3) which would leave you with 15 bars/pieces/slices etc. 1/3 would have to be divided into fifths. This would leave you with 15 pieces. Try using the 1/5 and 1/3 fraction bar. After you find the common denominator, you can proceed with adding or subtracting the fractions together.
1/3= _/15...3 goes into 15, ___ times 5/15
+1/5= _/15....5 goes into 15, ___ times + 3/15
1/3 and 1/5 canNOT be added unless there is a common denominator!