The third puzzle took 96 minutes.
Let x be the amount of time the first puzzle took. Then 2x is the amount of time for the second puzzle, and 2(2x) = 4x is the amount of time for the third puzzle.
We know that together they took 168 minutes:
x+2x+4x=168
7x=168
Divide both sides by 7:
7x/7 = 168/7
x = 24
This is the amount of time for the first puzzle.
The second puzzle took 2x=2(24) = 48 minutes.
The third puzzle took 4x = 4(24) = 96 minutes.