Question 409866
The normal setup for a problem like this is:

{{{t/A + t/B = 1}}}

t is the time they work together.  A and B are how long they would take working alone.  The 1 represents one complete job.  

For this problem, we let the first term be Gary...if if it takes Dad A hours, Gary takes 2A...

{{{(3/2)/2A + (3/2)/A = 1}}}
Now multiply by 2A and get
{{{3/2 + 3 = 2A}}}
{{{9/2 = 2A}}}
A = 9/4 = 2 1/4 hours
Therefore it takes Gary 4 1/2 hours alone...