How many 4-digit numbers are not divisible by 7?
from 1000 to 9999 is 9000 numbers
stating with 1001 and ending with 9996 there are 9000/7 =~ 1286 numbers ARE divisible by 7....
you can finish.....
YEE complementary counting!
so basically, what that is guest is you count the solutions you DONT want, then subtract. This is because it is sometimes faster this way.