I understand how to compute the HI tax but getting the OASDI tax is very hard. If I know that an employee was paid a semimonthly salary of $6,800 how can I find the OASDI and HI for the 9th paycheck, 16th paycheck, and 24th paycheck. I'm not understanding how to compute each of these

You need to figure out the gross wages earned through each pay peroid. For example the gross wages earned in the first semi-monthly pay period is 6,800. The second peroid or paycheck would be 6,800 * 2... See what you can out of what I posted if your still having problems post again.

$34000 including the 9th pay check, what next??