To answer this question you want to think about the possible combinations of containers. In particular (I'll start with (b), as it is the simpler case):

(b) The probability of 500ml (call this event A) is 0.2, which means the probability of <=500ml (call this event B) is (1-0.2)=0.8. So, for this part of the question, you want to consider the probability that all 10 containers have <=500ml, which is the probability associated with 10 instances of event B (hopefully you understand how to treat AND and OR in probability calculations?).

(a) In this part of the question, a number of combinations are acceptable, you could have 5 instances of event A (lets call this 5A for brevity), and 5 instances of event B (5B), or 6 instances of event A (6A) and 4 instances of event B (4B) etc. So, the entire set of scenarios is:

5A and 5B or

6A and 4B or

7A and 3B or

8A and 2B or

9A and 1B or

10A and 0B

I hope that makes sense.