Answer:
financing 62,400 dollars
Monthly Payment   $ 465.48 
Total Interest  21,386.4  
Rounding to nearest $ 100 
Additional $$  34.52
We save up to 16 payments and $2,136.4 in interest.
By-weekly payment  $232.60
Total Interest saved $ 194.4
Explanation:
78,000 less 20% down-payment: 62,400
Monthly  Payment
 
 
PV  $62,400.00 
time	180
rate	0.0034375
 
 
C  $ 465.484 
Total Interest
quota times time less principal
$ 465.48 x 180 - 62,400 = 21,386.4
$  500  -  $  465.48  =   $  34.52
 
 
C  $500.00 
time	n
rate	0.0034375
PV	$62,400.0000 
 
 
 
 

We now use logaritmics properties to solve for n

 -163.2956066
180 - 164 = 16 payments
Total Interst 500 x 163.30 - 62,400 = 19,250
Interest savings 21,386.4 - 19,250 = 2,136.4
If payment are bi-weekly:
then payments will be:
 
 
PV  $62,400.00 
time	360
rate	0.00171875
 
 
C  $ 232.598 
And total Interest:
232.2 x 360 - 62,400 = 21,192
Difference 21,386.4 - 21,192 = $ 194.4