Person A has 3000 kcal per day and reduces it to 2000 kcal he needs to loose 13 kg how long will it take him to loose the weight. ( Approx 7,716 kcal = 1kg.)
I am not confident in this answer. I'll give it a go.
Well he is losing 1000 calories a day, each of those 1000 of each day representing a fraction of the 7716 he needs to lose to lose 1 pound.
3000-2000=1000 7716/1000= 7.716 days which is how many days it takes to lose 1 kg, now how long does it take to lose 13kg?