(B100)-1 can be divided by 5; so unit digit of (B100-1) is either 5 or 0, so unit digit of B100 must be either 6 or 1, B99= B100-5, so unit digit of B99 is either 1 or 6, so the unit digit of B99-1 must be either 0 or 5, if and only if an integer has unit digit of 0, the integer will be divided by ten. If B99-1 has unit digit of 5, then B99-1 can not be divided by 10, for example 55,65,75,85,.....