Prove that:

a) 5^31–5^29 is divisible by 100.
The answer should be in this format: ____ * 100

b) 25^9+5^17 is divisible by 30.
The answer should be in this format: ____ * 30

c) 27^10–9^14 is divisible by 24.
The answer should be in this format: ____ * 24