21. It is not necessary to use a calculator as you already know 4 numbers which can be multiplied to make 17017 ( the final number). As those 4 numbers are all prime numbers the PFD would be 17017 = 7 x 11 x 13 x 17
22. Following the pattern, and additional 0 increases the incidies by 1, therefore, as 1 billion has 9 '0s', the PFD would be 1,000,000,000 = 5^9 x 2^9
