The 20th visitor will be the first to get both the key chain and the bumper sticker, I found this out by finding the least common multiple, or LCM, you do this by writing out the multiples of both 4 and 10, 4 8 12 16 20..... and 10 20 30... then you find the smallest number that both 4 and 10 have in common. I hope this helps, sorry if it doesn't.
