Is the offer irrevocable?

Answer yes or no and why:

6. Angie sends Ben a letter offering to sell Ben her house for $150,000 and stating that the offer will expire in 30 days if she does not hear from Ben in that time.

7. Sandy, a manufacturer of radios, sends Bob a letter offering to sell Bob 100 radios at $5.00 each and stating that it is a firm offer for 6 months.