1."Here's a funny four-figure number," said Charlie. "I've switched its first two digits, and also its last two." Mary shook her head. "You mean like changing 7439 into 4793? There's nothing special about that." "Yes," Charlie replied. "But in this case the result is exactly four times the original number." What was his number?

Charlie's number was 2641

Charlie's number was 1782

Charlie's number was 1528