A six?-digit code is used to distinguish between different regions. The code uses numbers between 1 and 7?, inclusive.
a. How many different codes are available for? use? (Assume that any six?-digit number can be used as a? code.)
b. The first three digits of a code for a particular region are 442
2. If no other region has these first three digits as part of its? code, how many codes can exist in this? region?