Solution:

Neither, the accountant was his sister.

Solution:

In Washington D.C.

Solution:

7 up cans

Solution:

No time at all it is already built

Solution:

1st person : Colin
2nd person : Rooney

They both lied.(I said at-least)
(If only one lied they would both be boys or both be girls)

Solution:

What

It stated 'WHAT' was the father's name.

Solution:

5625

n^1
75 * 75 = 5625 (sum of first 75 odd numbers).
50 * 50 = 2500 (sum of first 50 odd numbers).

Solution:

The mathematician made a small fence around himself and declared himself to be on the outside.

Solution:

Velma waits until night time and then goes through the second door.

(No sun at night)

Solution:

Mark Benson '?' = question MARK, so the note on the door reads 'Question Mark Crimson broke your window.'