All you need to do is add te averages together then divide them by 2.
91%+78%=169%
169%÷2=84.5%
So the new test average is 84.5%
306 is the total cards sent by all the students in the class. Lets say x is the number of students in class, therefore each student needs to send x-1 cards (since they do not send themselves a card). So depending on your level of math, you could either take the complex way to solve it using quadratic equations, or just find the number that if multiplied by itself then subtracted, gives you 306.

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Complex way

So a = 1, b = -1, c=-306
using the quadratic equation:

The description that accurately explains how a fair decision can be made in this situation is : IF a ball with number 4 and 5 are picked you order the largest size, if a ball with number 4 and another with number 4 is picked you order the medium size, but if the balls picked are that of number 2 and 4, then you order a small size. The probability of each result occurring is a fair one.
<h3>Meaning of probability and full working</h3>
Probability is the degree of occurrence of an event. it can also be said to be the chance for an event to occur.
Probability helps us know the chance an event has to occur. The closer it gets to zero(0) the less chance it has to occur. The closer it gets to one(1) the more chance it has to occur.
Learn more about probability: brainly.com/question/14868936
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Answer:
0.046231
just use a calculator my dude