Shutdown system terminates the chain reaction in a reactor core immediately following quick insertion of shutoff rods in the core. Reactivity worth of shutoff rods should be adequate enough to overcome positive reactivity excursion following a design basis event (DBA), offset required power defect and render adequate subcriticality. For a given core, if positive reactivity insertion (in a U-235 fueled core) following a DBA and total power defect is one and two dollar, respectively, estimate the available subcriticality margin following the drop of shutoff rods if the ratio of neutron counts before and immediately after shutoff rods insertion is 10 .