Mathematics · Levelling Calculations

A levelling instrument has been set up at a station with a height of instrument (HI) calculated from a bench mark having an RL of 115.385 m. If the staff reading taken upside down (inverted staff) at the bottom of a bridge deck is -2.835 m and the height of instrument is 116.705 m, what is the RL of the bottom of the bridge deck? [Note: Base data corrected for standard inverted staff convention: RL of station = 115.385 m, staff reading = -2.835 m, instrument height above station = 1.320 m]