# Thread: A Biology question I am stuck on, uses math.

1. ## A Biology question I am stuck on, uses math.

Assume that one square kilometer of field produces 300 tonnes/kmē (1 ton =1000kg) of grain every year. The grain grown in that field contains 141900kJ of energy. Assume that an average person needs 2400kJ every day of energy to survive.

I am not sure how I would go about calculating when it says:

Calculate the number of people a 1kmē field could support for one year if people ate only grain. The options are as follows:

a) 125
b) 4.86 x 10^4
c) 1.77 x 10^7

Thanks for the help! I really appreciate it

2. Originally Posted by M23
Assume that one square kilometer of field produces 300 tonnes/kmē (1 ton =1000kg) of grain every year. The grain grown in that field contains 141900kJ of energy. Assume that an average person needs 2400kJ every day of energy to survive.

I am not sure how I would go about calculating when it says:

Calculate the number of people a 1kmē field could support for one year if people ate only grain. The options are as follows:

a) 125
b) 4.86 x 10^4
c) 1.77 x 10^7

Thanks for the help! I really appreciate it
Right, the average person needs 2400kJ a day, so every year they would need $\displaystyle 2400 \times 365$kJ (I presume we won't take into account leap years here?)

The number you are left with will be the amount of energy that one person consumes a year, there if you divide that amount by the amount produced by the field in a year, you will get the number of people:

$\displaystyle \frac{(2400 \times 365)}{141900kJ }$

Hope this helps

3. Thanks...but, using that, it seems right, but it does not match the options....

4. Originally Posted by M23
I posted this before, but got an incorrect answer. I really do not know how to go about doing this calculation. If someone can just explain to me how I should do it, it would be very much appreciated.

Assume that one square kilometer of field produces 300 tonnes/kmē (1 ton =1000kg) of grain every year.

The grain grown in that field contains 141900kJ of energy. Assume that an average person needs 2400kJ every day of energy to survive.

I am not sure how I would go about calculating when it says:

Calculate the number of people a 1kmē field could support for one year if people ate only grain. The options are as follows:

a) 125
b) 4.86 x 10^4
c) 1.77 x 10^7

Thanks for the help! I really appreciate it
The grain grown from 1 Sq.Kilometer:

$\displaystyle \frac {141900kJ}{2400kJ} = 59.125$ days.

The field would support 1 person for 59 days!
That means that a person would have to consume 300 tons of food to survive for only two months. Not logical (to me).

Something is wrong or missing in your post.
Most cereals have a caloric value of about 3000 to 4000 calories per kilogram. A food calorie (the kilocalorie) is approximately 4.18 kilojoules, which means that kilogram of grain has somewhere between 13,000 and 16,000 KJ of energy. The grain from your field is producing 10 times the expected value! [Let me know where this is; I want to buy the land.]

If you meant to state that each kilogram from the field produces 141,900 KJ of energy, then you have a better chance.

Would you please check the data in your post, and if needed edit it?

Note: 300 tonnes/kmē approximates 100 bushels per acre (for some grains) so I can swallow that.

5. You were right about the kJ/kg idea. It is in fact 141, 900 kJ/kg. Thanks.