3. Zack missed the way: 0 boxes, 53 pots.
4. No. 53 is not evenly divisible by 10.
5. The number of ways Zack can plant his flowers is 1 more than the number of 10s in the number of flowers he has. If Zack has 10a +b flowers (0 ≤ b ≤ 9), then he can plant them (a+1) ways.