Hi Alex,
This depends on two things:
1) the size of the original hole;
2) whether your "bum" is too large for your trousers.
I have torn through the backside of my grey trousers several times, so of course, I feel your pain.
Particularly when I do not notice the hole before I go to work in the morning, so my work colleagues make fun.
All I can suggest is that you get a needle and perhaps a patch (preferably a matching colour to your trousers) and sew that bad boy up.
Alternatively, splash the cash and get some new trousers.
Remember, as my father always told me - "dress for success" or was it, "dress to impress"? I can't recall. No matter, I've just realised, I myself have re-torn the backside of my pants.
Off to the tailor, I go.
love forever - Matt Gould.
Matt Gould, March 2012