A rule to calculate the cooking time in minutes for a joint of meat is:
Multiply the weight in kilograms (kg\text{kg}kg) by 40 then add 25
A joint of meat weighs 3.5 kg.
Work out the cooking time, in minutes.
The cooking time for a different joint of meat is 105 minutes.
Work out the weight of this joint of meat, in kilograms (kg\text{kg}kg).