To find out if a number is divisible by seven

Take the last digit, double it, and subtract it from the rest of the
number.
If the answer is more than a 2 digit number perform the above
again.
If the result is 0 or is divisible by 7 the original number is also
divisible by 7.
Example 1 )   259
9*2= 18.
25-18 = 7 which is divisible by 7 so 259 is also divisible by 7.
Example 2 ) 2793
3*2= 6
279-6= 273
now 3*2=6
27-6= 21 which is divisible by 7 so 2793 is also divisible by 7 .
Now find out if  following are divisible by 7
1) 2841
2) 3873
3) 1393
4) 2877