They will arrive at the beach at 8:05 AM.
Let's break this down step by step.
At 7:05, Mr. Mallery says they are one-third (1/3) of the way there.
At 7:25, Mrs. Mallery says they are 75% of the way there, which is 3/4.
The difference between these two updates is the distance they traveled in 20 minutes (from 7:05 to 7:25).
They went from 1/3 to 3/4 of the way in 20 minutes, which means they covered another 2/3 of the remaining distance in that time (3/4 - 1/3 = 2/3).
Since this 2/3 of the distance was covered in 20 minutes, we can figure out how long it would take to cover the whole distance:
1 (total distance) = 2/3 (covered in 20 minutes) * (60 minutes / 20 minutes)
1 = 2/3 * 3
1 = 2
So, they will take 60 minutes to cover the whole distance at their constant speed.
Now, let's calculate the time:
They started at 7:05 and will take 60 minutes to reach the beach, so:
7:05 + 1 hour = 8:05
They will arrive at the beach at 8:05 AM.