Investigate division by \(9\).
a) Find the quotient and remainder for \(79\div9\), \(80\div9\), \(81\div9\), and \(82\div9\).
b) How many different remainders are possible when dividing by \(9\)? List them.
c) Why can the remainder never be \(10\) when dividing by \(9\)?
Hints
- Start with a nearby multiple of \(9\) for each dividend.
- Imagine putting the leftovers into another group of \(9\). When would that be possible?
- Test several possible leftovers to decide where the list of remainders must stop.
Solution
1. \(79=9\times8+7\), \(80=9\times8+8\), \(81=9\times9+0\), and \(82=9\times9+1\).
2. There are \(9\) possible remainders: \(0,1,2,3,4,5,6,7,8\).
3. If \(10\) were left over, another full group of \(9\) could be made, leaving \(1\) instead. The remainder cannot be as large as the divisor.
Answer
a) \(8\) remainder \(7\); \(8\) remainder \(8\); \(9\) remainder \(0\); \(9\) remainder \(1\).
b) \(9\) remainders: \(0,1,2,3,4,5,6,7,8\).
c) Another group of \(9\) can be made from \(10\) leftovers, so \(10\) cannot be the remainder.