Sodapop is correct, however in SIMPLISTIC terms:
F20 generally better when runway length limited
F10 generally better when obstacle limited
Note that you GENERALLY become length limited for a given thrust setting before becoming obstacle limited - therefore F20 is USUALLY generated from the computer if the optimum [default] setting is requested.
ie: at low weights/long runway F10 MAY be generated. at High weight/ shorter field length F20 is USUALLY generated.
That's enough for the interview.
Note: the PILOT can elect to override the optimum setting by inputting either 10 or 20. For one example; x-wind conditions at max thrust derate [lowest takeoff thrust setting] , the pilot may elect to use Flap[s] 20 to provide a further buffer against possible tailstrike.
Hope that helps b8z