Answers

2014-09-12T10:34:10+08:00
The formula for the average of n values would be:

(Value1 + Value2 + ... + Value n) ÷ n = Average

So the equation would be:
(83 + 80 + 85 + 77 + (5th grade)) ÷ 5 = 83

Let's rearrange to get the 5th grade. Multiply both sides by 5.
(83 + 80 + 85 + 77 + (5th grade)) = 83 * 5

Let's simplify both sides:
325 + 5th grade = 415

Then subtract 325 from both sides so that only the 5th grade would be left.

5th grade = 415 - 325

The 5th grade is 90.

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2014-09-12T21:56:37+08:00
For  shorter solution, you simply

83 x 5= 415 - this includes the fifth grade

then

83 + 80 + 85 +77 = 325 - add the four grades

lastly,

415-325 = 90

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