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# Four numbers are displayed in a row. The average of the first two numbers is 8, the average of the middle two

numbers is 11.8, and the average of the last two numbers is 7.7. Determine the average of the first and last number in the row.

### 2 Answers

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- physicistLv 41 decade agoFavorite Answer
Let a, b, c ,d be the numbers; then you have

(a + b)/2 = 8

(b + c)/2 = 11.8

(c + d)/2 = 7.7

Adding the first to the third equation and subtracting the second you have

(a + b)/2 +(c + d)/2 -(b + c)/2 = 7.7= 8+ 7.7 - 11.8 ==> (a + b + c + d - b - c)/2 = 3.9 ==> a + d)/2 = 3.9 = average of of the 1st and last numbers in the row.

- SwamyLv 71 decade ago
Let a, b, c, and d be the four numbers such that

a + b = 8 X 2 = 16

b + c = 11.8 X 2 = 23.6

c + d = 7.7 X 2 = 15.4

Needed to find out a + d divided by 2.

(a + b) + (c + d) - (b + c) = a + d

16 + 15.4 - 23.6 = 31.4 - 23.6 = 7.8

Average is 7.8/2 = 3.9

Average of first and last numbers is 3.9

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