Answer: The maximum the average can be for all the guests at the birthday party is 2 minutes 57 seconds.
Explanation:
Since the ratio of adults to children is 4:1, then it follows that:
4/5 of the people are children
1/5 of the people are adults
A weighed average of the times would give:
1 minute 25 seconds = 85 seconds [for 1/5 of the people (the adults)]
3 minutes 20 seconds = 200 seconds [for 4/5 of the people (the children)]
(85 x 1/5) + (200 x 4/5) = 177 seconds
Therefore, the maximum the average can be for all the guests at the birthday party is 2 mins 57 secs.