Here are the divisibility rules to determine if a number is a factor or not.
Divisibility by 7: This one is a little complicated. To determine if a number is divisible by 7, take the last digit off the number, double it and subtract the doubled number from the remaining number. If the result is evenly divisible by 7 (e.g. 14, 7, 0, -7, etc.), then the number is divisible by seven. This may need to be repeated several times.
Example: Is 3101 evenly divisible by 7? 310 - take off the last digit of the number which was 1 -2 - double the removed digit and subtract it 308 - repeat the process by taking off the 8 -16 - and doubling it to get 16 which is subtracted 14 - the result is 14 which is a multiple of 7
310 - take off the last digit of the number which was 1 -2 - double the removed digit and subtract it 308 - repeat the process by taking off the 8 -16 - and doubling it to get 16 which is subtracted 14 - the result is 14 which is a multiple of 7