Find the digits a and b such that the six-digit number 543ab2 is divisible by 42. (Do not use a calculator for this problem, and be sure to explain how you found your solution.)
Let N be a three-digit number whose digits are all distinct. There are nine two-digit numbers that can be formed using the digits of N. Assume that N is the sum of all nine of these two-digit numbers. Find all possible values of N.