Question

how do i complete the rest of the table

Answer 1

To find Vanilla for ages 8-12, you add up the known amount from each flavor for that age group and subtract it from the total to get... 50 - (25 + 12) = 13
To find Chocolate for ages 13-17, you add up the amount of people know to pick up late and subtract it from the total amount of people who picked chocolate to get... 100 - (35 + 25) = 40
Now that we have that number, we can use it to find the amount of people ages 13-17 who chose Vanilla. To find this you subtract the total amount of people who chose chocolate and strawberry from the total amount of people to get... 80 - (40 + 12) = 28

This should help :)

Answer 2

There is insufficient information. You have 7 blanks and only 6 independent equations.

By making the numbers add up, the two middle rows can be filled as
.. 25 .. 13 .. 12 .. 50
.. 40 .. 28 .. 12 .. 80

For the other four emply slots, you can pick one value and compute the rest based on that. If there is additional information, such as Vanilla = Strawberry in young ages or in total, for example, then you can use that to find the remaining values. Otherwise, you're out of luck.