Nitrogen has an average atomic mass of 14.00674. it has two isotopes, n-14 and n-15, with atomic masses of 14.0031 amu and 15.001 amu, respectively. what is the abundance of n-15?

In this case, we are going to assume that there are 100 atoms to make things easier. 

Let R% be the abundance of n-15. With this in mind, we calculate the abundance of n-14 to be 100%-R%

14.0031*(100-R)% + 15.001 * R%= 14.00674 
In this case, we can delete or ignore the % sign since we do not want to carry it around, however, we need to keep in mind that the final answer is in %

14.0031*(100-R) + 15.001 * R= 14.00674 
1400.31-14.0031R+15.001R=1400.674
0.9979R=0.364
R=0.3648

Then, the abundance of n-15 is 0.3648%

The answer to your question really depends on three things: how much water you have, how cold it is to start out, and how cold the things around it are. water actually freezes when it gets to 32 degrees fahrenheit (0 degrees celsius), but the time it takes to get there may be different.  let's start with the first. if you take two glasses, and fill one with a tiny bit of water, and the other about halfway, then put them both in the freezer, the one with less water will freeze first (you can try this at home, but i recommend using plastic cups and not glass ones).  now let's move on to the second part. let's say you have two glasses, and you fill one with really cold water that has been in the refrigerator, and the other with really hot water from the sink. if you put both of them in the freezer, the one that started out colder will freeze first.  for the third part, let's imagine that you have two glasses with the same amount of water in them, and the water is at the same temperature. imagine putting one outside on a really really cold day in georgia, and having a friend in alaska put one outside on the same day. since it would be so much colder in alaska, the glass of water there would freeze before yours.  so, if you took a tiny bit of really cold water in a glass, and put it outside on a cold day in alaska, it would freeze a lot faster than a big glass of hot water outside on a cold day in georgia. 

Combustion is the act or process of burning. therefore, i believe the answer is  a: burning wood.