SRAM ream derailleur replacement X3 for X5
 

Trying to fix a neighbor kids bike cheaply. Can I replace a broken SRAM X5 with a new SRAM X3 on a 9-speed mountain bike? They both appear to be 1:1 activation ratio.

Thanks

Yes, all Sram derailleurs with an X.n designation are plug compatible. Generally folks upgrade when replacing, but you can go either way.