We are looking for a number 97xxxxxxxxx97 with s.o.d.=99, meaning that the missing digits sum up to 99-32=67 which is 4+7*9,- so the minimal choice would be xy9999999, where x+y=4.
Out of 3 candidates to be tested for divisibility by 99(97229999999,97139999999, 97319999999) the last one fits. **
**Luckily, the answer came out quickly,- if not, we should have tried xy8999999, where x+y=5 etc
Edited on July 20, 2016, 11:56 pm