Respuesta :
The first and last number usually (apart from 99 and a few others) add up to nine.
eg: 9X9=81
8+1=9
9X6=54
5+4=9
9X13=117
1+1+7=9
get the idea :)
eg: 9X9=81
8+1=9
9X6=54
5+4=9
9X13=117
1+1+7=9
get the idea :)