We can justify this in 2 ways:
1. Think about what 28 x 101 means. It means that you have 28 added together 101 times. Therefore if you have 28 added together 100 times and you add it again, it means 28 added together 101 times.
2. 28 x 101 = 28 x (100 + 1) = 28 x 100 + 28 x 1 = 28 x 100 + 28