Maths solving !!

Amber’s grandmother is cooking a roast for their Sunday dinner. It weighs 4 kilograms and needs to be cooked for 40 minutes for every kilogram of weight. She puts it in the oven at 9.50 am. What time will the roast be ready?

How I got the answer

4x40= 160

160min 2hr 40min

So the roast started cooking at 9:30 

And it took 40min but you have to add the 

4 kilograms because it doesn't take 40min for the whole roast to cook cause you have to cook the roast parts 4 for times and 4x40= 160 and if you plus 9:30 and 2hr + 40 min that will be 2:40 and that when the roast will be ready.