Rex is currently taking physics as one of his electives in school. his grade at the end of the year is determined by the average of four exams, each worth 100 points. on his first test he got an 84, on his second he got a 86, and on his third an 92. what must he score on his fourth exam to get an a (90%) in the course?

Marks scored by Rex in his three tests are 84 ,86 and 92. Let marks scored by Rex in his fourth test be x.

Average of the marks =

Given average of all four marks is 90.

Forming an equation.

Solving for x.

Cross multiplying we have:

262+x=360

Subtracting both sides by 262

x=360-262

x= 98.

Answer :Rex must score 98 marks in his fourth test to have an average of 90 % .

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! )