Lois wants to average 83% on her 5 tests to assure herself a B. If she scores 72%, 90%, 85% and 88% on her first four tests, what does she need to earn on her last test to meet her goal?

83

Step-by-step explanation:

72+90+85+88=335

335 ÷ 4 = 83.75

335+83=418

418 ÷ 5= 83.6

the answer   is 200

step-by-step explanation:

this is a problem about poisson probabilities.   i don't know if you've

already studied poisson distributions, or if you have a place to look

to read more about the subject.  

here's how you can see a poisson distribution coming.   when there are

a lot of events that may happen at any time (or any place), and when

you're counting how many of them happen at a particular time (or a

particular place), and each event is independent of the others, then

the result is a poisson distribution.   this is roughly true for the

chocolate chips; one chip being in a cookie has only a small effect on

the probability that another chip will be in the cookie.

once you know you have a poisson distribution, you know a lot.   there

is only one parameter to a poisson distribution, so if you know the

mean, you know everything.   the form of the distribution is

  probability of n chips = {[e^(-x)]*(x^n)} / (n! )