Chance of winning

[Gene2x]

How do you calculate your hand's chance of winning on the flop? For instance, you hold QJ, have six opponents (random cards), and the board is 6-8-5. Software tells me my equity (chance of winning) is about 7%.

Is there a quick way to ballpark the answer using some simple math? Or, do you have to know basic Hold'em statistics and then do math using those numbers? Is it even possible to do within 30 seconds before your PartyPoker timer expires?

Thanks,
Gene