1. Don put down his pen. “I just worked out something about your Dad’s phone number.” “What’s that then?” said Mary. “Simple. If you exchange its two parts, putting the 4-digit part first, you get one more than double his number. That’s one more than double the complete 7-digit thing.” What was it?

Phone number 435-8717

Phone number 427-9012

Phone number 139-4651