Answer:
The 15th visitor
Step-by-step explanation:
You have to find the LCM of the two numbers given by multiplying the chances that a visitor gets either a bumper sticker or a key chain.
1/3 x 1/5 = 1/15.
This means that every 15 people that visit the museum get both the bumper sticker and the key chain but since 15 is the lowest positive multiple of itself, the 15th visitor to the museum every day will be the first to get both items.