Every fourth visitor to a museum gets a free bumper sticker every tent visitor to the museum gets a free keychain which visitor will each day will be the first one to get both the bumper sticker and the keychain

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.