For a positive integer n, define f(n) to be the number of non-empty substrings of n that are divisible by 3. For example, the string "2573" has 10 non-empty substrings, three of which represent numbers that are divisible by 3, namely 57,573 and 3. So, f(2573)=3. How many five-digit numbers n have f(n) = 6? Solution