This lesson is the first time that students must add or subtract fractions that do not have the same denominator. Limiting our work to denominators 10 and 100 makes this new concept accessible. We start by physically using base-10 blocks to explore tenths and hundredths, then we use the blocks to add and subtract, next we draw the base-10 blocks, and finally we practice the concept abstractly.
The abstract algorithm is similarly straightforward. For example, to add 6/10 + 15/100, students first change 6/10 into 60/100. Then it’s easy to find the sum: 60/100 + 15/100 = 75/100.