A word formula is an instruction that tells you how to work something out.
Here is a word formula to work out the cooking time (in minutes) for roast beef. Using this formula, the beef will come out cooked "medium".
Cooking Time in minutes = Weight in kg x 30
For how long should a 3 kg joint of beef be cooked?
The weight is 3 kg, so ....
Cooking Time = 3 x 30 = 90 minutes