The reason there will be a 6 in the hundredth's place is that there is nothing in the hundredths place in the minuend (number we are subtracting from). We can consider this a zero. Since the subtrahend (the number we are subtracting) has a 4, only 4 + 6 could give us the zero.
8.50
- 4.64 - notice that only a 6 could give us the zero in 8.50.