Kate's model is correct because 10 times the value of 4 rods, 10(4(10)), is 400. Mel's model represents 4(1)=4, which is incorrect. Kate's model represents 4*100=400, which is correct.
Hope I answered your question!
I don't know about the first one but for the second one:

<span>The first person can have any birthday. That gives him 365 possible birthdays out of 365 days, so the probability of the first person having the "right" birthday is 365/365, or 100%.The chance that the second person has the same birthday is 1/365. To find the probability that both people have this birthday, we have to multiply their separate probabilities. (365/365) * (1/365) = 1/365, or about 0.27%.</span>
1.57, divide two times pi (3.14)