Seemingly simple conditional macro

My goal is for one of the following equations to occur:

If 3d6>10 then subtract 10 from the original (3d6) roll.

If 3d6>10 then divide original (3d6) roll by 2 and round down. 

So far I don't understand how to do it. 

Note: you have done > in both cases above, should they be different?

It doesnt matter though, because you cant do conditional branching in roll20 macros. 

There's no way to modify a roll based on that roll's value without using the API (a Pro subscriber perk). If your GM has Pro, a script could be written to do the above, but it's not as easy as creating a macro.

For really simple conditionals, you can sometimes fake it with roll20s keephighest and other functions, but this isnt one of those cases.

Could you use !hitormiss and $[[0]]?

OP isnt a pro user so i didnt suggest that, but if they were pro users hitormiss is one way to do it.

The $[[0]] trick wouldnt work because you cant do anything with the object, you cant halve it or subtract 10 from it.