Should I cut my 100 m rope into 70 m/ 30 m, 80 m / 20 m, or 60 m/ 40 m?

I do a lot of climbing at city of rocks so a 80 m rope would be handy for the 140 ft routes.  But a 70 is always helpful.  And a 20 m rope is pretty useless.  

The real question is why did you buy a 100 meter rope?

Giles LaJevic-Augustine wrote: I do a lot of climbing at city of rocks ...

It’s really dirty there. Have a plan in place to wash it correctly before cutting it.  I recommend avoiding Tide Pods.

Don't cut it until you need to trim a few meters off the end due to wear. You'll be able to trim several times and still have a nice long line for climbing all your favorite lines. Plus, if you want to go to Norway to climb in the cave you'll be all set!

Its pretty simple,  I got a good deal on a 100 m rope, cheaper than a 70, and I wanted to replace my 70 m. I thought i could use the 30 for the gym or glaciers,  its the new bd rope that is a single double and half.   

Ive been climbing at the city for yeara and dont really have a problem with it being dirty there.  Especially at castle rock when you are on those multi-pitch routes.  

Years ago I had 100m rope that a rock landed on which created a core shot. It became a ~75m/~25m rope. A friend took the the long end and I use 25m for canyoneering.

70+30 or not at all.  I'd start with "not at all" and cut if/when you get tired of flaking/coiling it.

30m might not be that useful on a glacier if the falling in the slot part doesn't go very well.  70m is ideal.

But how about chopping the 30m into a bunch of dog leashes that you can then hand out for free as needed at the City/Castle Rock???

Now THAT would be a good use of the 30m!!!!

Cut it or don't cut it according to your needs. Why ask strangers?

Don't cut that cord until you need to. Use a shorter rope & a rope bag to climb around CoR.
 Save the long rope.  Go use it to its full potential. Go climb something on the War Bonnet or Elephants Perch or the Selkirks.

You should only be cutting ropes on lead, so how would you know what length it'll be?

In seriousness, for where I live, having a 70 and 30 makes the most sense. I guess it really depends on what kind of climbing you do.

Giles LaJevic-Augustine wrote: I do a lot of climbing at city of rocks so a 80 m rope would be handy.  But a 70 is always helpful.  

You already answered your question, the 80 works when you need a 70 but not vice-versa. A 30 is marginally useful anyway but easy to come by used or factory shorts if you want one. Or save and use the 100 for when you need more than 70 and have ability to chop the ends when needed. One rope is never enough.