Q: Hey bill if you wanna make your life easier when it comes to figuring out if something is a multiple of 7, there's a convenient way to use your subtract/add a multiple of 7 trick that makes it much easier.   Say your number is 30583 for example.   Now we know that 21 is a multiple of 7 right?   That means anything times 21 is also a multiple of 7.   So what we can do is just subtract the number by 21 times the last digit of the number, in this case 3.   So 21*3=63 which means we can do 30583-63->30520 then divide by 10 to get 3052.   Then repeat.   3052-42->3010->301 301-21->280->28 which is divisible by 7 which means 30583 is divisible by 7.  Now whenever we are subtracting by 21 times the last digit and then dividing by 10, what we are essentially doing is taking the last digit, doubling it, then subtracting the rest of the number by t
 he doubl
 ed last digit.   So like instead of doing 30583-63->30520->3058 what you can instead do is a short cut for 30583 and take the last digit 3, double it to get 6, and then subtract the rest of the number by that doubled digit, 3058-6->3052, and then just keep subtracting the rest of the number by the last digit doubled until you get to a number you know is divisible by 7.
A: i would just go to 3059, 301, 35. so 437