Okay, so there's a number rounded to the nearest million, in this case 2 million. So this tells me that the number of visitors is between 1,500,000 and 2,499,999. The reason why 1,499,999 doesn't count is because the hundred thousands digit is not 5 or above, so it doesn't count. It would round down to 1 million, not 2 million. The reason why 2,500,000 doesn't count, is because the hundred thousands digit is 5, so that rounds to 3 million, not 2 million. The greatest number of visitors that visited the aquarium has to be 2,499,999, because that's one digit away from 2.5 million, and the hundred thousands digit is a 4, not 5. The answer is 2,499,999