Question 173176
Try this!
Starting with one cell at time zero and doubling every 4 minutes:
Time-----------Cells
..0(4).........1 --> {{{2^0 = 1}}}
..1(4).........2 --> {{{2^1 = 2}}}
..2(4).........4 --> {{{2^2 = 4}}}
..3(4).........8 --> {{{2^3 = 8}}}
.
.
..45(4).........{{{2^45}}}

So you need to know how many times this doubling will take place in 3 hours.  Divide 3 hours, that's 180 minutes by 4 to get 45 times, (n = 45) so at the end of 3 hours, there will be {{{highlight(2^45 = 35184372088832)}}} cells.