an carnot engine absorbs heat at high temperature 800k and efficiency of the carnot engine is 50% what is the high temperature to increase efficiency to 80% if the low temperature kept constant ?

Efficiency n = (t2-t1)/ t2 

Where t2 is the source temperature and t1 is the sink temperature  

Now when the efficiency is 50% 

0.5 = (800-t1)/800

t1 = 400K 

When the efficiency is  

0.8 = (t2-400)/t2 

The required temperature of the sink will be  

T1 = 2000k