Assuming that you would pay off the debt now and not wait any longer than to calculate how much more money you need you simply need to subtract the $1000 that you have from the total which is $2400.
$2400 - $1000 = $1400
Finally, we can see that you need an extra $1400 in order to finish paying off the debt. Unless you wait more time and the overall debt owed rises some more.
