It is your sister's birthday and you are preparing a family meal. the joint of meat needs to be cooked for 30 minutes per 500 g plus an extra 20 minutes. how long will it take to cook a joint of meat weighing 2.5 kilograms?

It will take 170 minutes or 2 hours and 50 minutes to cook a joint meat weighing 2.5 kilograms.

Given:
30 minutes per 500 grams plus an additional 20 minutes
2.5 kilograms join meat

1 kilograms = 1000 grams

2.5 kg * 1000g/1kg = 2500 grams

30 / 500 = x / 2500
30 * 2500 = 500x
75000 = 500x
x = 150 mins 

Cooking time of 2.5 kg joint meat is 150 minutes plus an extra 20 minutes to a total of 170 minutes or 2 hours and 50 minutes.