Assume you are overweight (205lb) and want to loose 10 pounds of body
fat. As we know, one pound of body fat contains 3,500 calories. How
long will it take to loose the ten pounds of fat if you do aerobic
exercise everyday for 2 hours? *Note: A 205lb. person will burn about
10 calories per minute during aerobic exercise.

Your question provides all the necessary information for the answer.

If 1 lb of body fat contains 3,500 calories and this person wants to
lose 10 lbs, then they really want to burn 35,000 calories.

If a 205 lb person burns approximately 10 calories per minute doing
aerobic exercises, then to burn 35,000 calories would take 3,500
minutes.

3,500 minutes = 58 hours and 20 minutes = 2 days, 10 hours and 20
minutes

This, of course, is assuming that the burn rate of calories doing
aerobic exercise is approximately the same as the person loses weight
from 205 lbs to 195 lbs. It is also assuming that no additional weight
is gained during the exercise program.

So if this person is doing aerobic exercises for 2 hours each day,
then it would take 30 days to lose the ten pounds of fat where the
30th day would only require 20 minutes of aerobic exercise.

Assume the question-asker wants someone else to do her homework...

For every pound of muscle your body gains or loses, your muscle
metabolism is affected by 35-75 calories per day depending on
genetics. calorie burn would drop as weight was lowered, lean tissue ,
muscle and water would make up about 10% of this 3,500 calories burnt.

Assuming no dieting, which often results in muscle loss, aerobic
exercise would never increase fat loss to the same levels as weight
training due to the increase muscle gain through weights and the
reulting increase in metabolism past the duration of exercising.

The thought of running 2 hours a day, compared to avoiding eating a
few mars bars seems to show that dieting benefits fat loss, along with
weight training to increase muscle mass (with shorter exercise periods
than aerobic)

1200 calories over two hours, even if you weighed 220 pounds then
running at 8mph would burn 20.0 per minute. Meaning running constantly
for one hour at 8mph or walking for 2 and a half hours each day
burning 8.0 calories per minute.

Lowering your calorie intake by 1000 calories per day would save 2
hours of continous flat out exercise. As the person wants to lose body
fat they are probably not physically fit, running for an hour seems
unlikely whereas not eating Big Mac and Super-Size fries at 1100
calories.

Weight training for 30 minutes per day plus lowering calorific intake
would make a person lose fat and increase metabolism. Exercising for
two hours aerobically without rest days would cause damage to the body
very quickly.

The first anwser is absolutley correct, this is more of a general
reply. It is never as simple as 3,500 calories burnt, one pound of fat
lost. This is before we get started on Structural fat, Adipose tissue,
ketones, dehydration and so on.

Who is kallisti-ga? And why is this person concerned about the question asker?

I thought I would add a few notes to add to the excellent information
already given on this question. One important issue to consider here
is actually determining the body fat level of a person before you can
correctly guage when 10 lbs of fat has been lost. When beginning a
reduced calorie diet for example, people who are very obese often have
dramatic weight loss in the first two weeks. This is largely water
loss and not all fatty tissue loss of course.

Drastic changes in diet and lifestyle and/or exercise routines can
cause losses of 20 lbs in two weeks, however, weight loss of 1.5 lbs
per week is really typical if one sticks to a diet. A loss of more
that 3lbs per week can actually be unhealthy for many individuals.
Weight alone cannot be the determining factor in guaging weight loss
progress; the body fat ration must also be taken into account.

Additionally, body type is a factor. If someone is 5' 2" with a thin
bone structure and weighs 205 lbs, they will require - generally
speaking - a different approach to weight loss than someone who is 5'
8" with a heavy bone structure, as their healthy weights are not the
same, and the exercises best suited for each will vary as well. The
person in the first instance would likely find any aerobic activity
which placed impact on the feet and knees difficult, while such
activities might not affect he person in teh second instance.

It's not reccomended that anyone undertake a diet of less than 1200
calories per day if a woman, and about 1500 per day if a man. Larger
framed people can and probably should increase those minimums by
200-300 calories. Muscle deterioration becomes a possibility
otherwise. What is highly suggested is exercise to go along with the
dietary intake monitoring. Swimming is the single best exercise,
exercising more muscles in the body than any other. IT has the
additional benefit of bouyancy, which takes weight off joints,
creating virtually impact free aerobic exercise. Resistance exercise
can be achieved with commercially available hand paddles and fins such
as used for snorkeling.

In summary, an extremely obese person can lose 10-15 pounds of fluids,
4 lbs of fatty tissue and a few pounds of muscle if they don't
exercise, and have lost upwards of 20 lbs. However they won't have
lost 10lbs of fat. Excercise is simply a good thing, it increases
metabolism, and builds muscle tissue - which burns calories even at
rest, creating a synergystic effect with your diet and lifestyle
changes. There are several ways to determine body fat ratios; calipers
can be purchased, or many medical facilities can use immersion
methods.

I hope this might add some ideas to think about.