Question 1126204
Note that four of 15 minutes make one whole hour.
3 hours is three multiplied by four; to count number of 15-minute time periods.



A few parts here are omitted but you can fill them in yourself.
the  character, ^, means that the number after it is as an exponent.
<pre>
Number                    Expression for
of                        Number of           Exponential 
15-Minute Periods         Bacteria            Form
0                          5
1                         5*3                 5*3^1
2                         5*3*3               5*3^2
3                         5*3*3*3              .
4                         5*3*3*3*3            .
5                         5*3*3*3*3*3          .
.
.
12 (three hours)          {{{5*3^12}}}           {{{5*3^12}}}
</pre>