The c-14 decay is used to estimate the age of an old dead tree. the activity of c-14 in the dead tree is determined to have fallen to 21% of its original value. c-14 has a half-life of 5700 years. calculate, in years, the age of the dead tree.
i used the relevant formula but somehow my answer does not match with the markscheme. can you me?

Air enters a gas turbine with two stages of compression and two stages of expansion at 100 kpa and 17°c. this system uses a regenerator as well as reheating and intercooling – the intercooler returns the air to the inlet temperature. the pressure ratio across each compressor is 4 ; 300 kj/kg of heat are added to the air in each combustion chamber; and the regenerator operates perfectly while increasing the temperature of the cold air by 20°c. determine the system’s thermal efficiency. assume isentropic operations for all compressor and the turbine stages and use constant specific heats at room temperature. (0.378)