Time taken to cook 500 g or half kg of meat =30 minutes
time taken to cook 1 kg of meat = 30/0.5 = 60 minutes
time taken to cook 2.5 kg of meat = 2.5* 60 = 150 minutes
Now extra 20 minutes are also required for cooking.
So total time taken to cook 2.5 kg of meat = 150 minutes + 20 minutes
=170 minutes
Answer is time taken to cook a joint of meat weighing 2.5 kg is 170 minutes.